Joint effort by @ohlidalp, @tritonas00, @AnotherFoxGuy, @CuriousMike56

⬆️ Updated mygui, pagedgeometry and caelum
🐛 Fixed missing MyGUI shaders
🐛 Fixed MyGUI rendering issue
🔧 Build OGRE with resourcemanager_strict=off
updated rtshader media
🐛 Fixed cmake not accepting `CMAKE_CONFIGURATION_TYPES`
fixed odef lights scenenode
🐛 Fixed Caelum not loading os files
Caelum time slider: restored original code (it works with latest Ogre master branch)
🐛 Fixed Imgui inverted colours on DirectX 9
fixed survey map texture updating
⬆️ Update OGRE to 13.5
updated rtshader media for Ogre 13.5
🔧 Implement PSSM3 properly
⬆️ Update OGRE to 13.5.1
Fixed crash on loading 2nd terrain w/envmap enabled
⬆️ Update OGRE to 13.5.3
Fixed crash on 2nd map load (procedural road material conflict)
Media: fix corrupted DDS texture crashing under D3D9

Status: builds and runs OK, but there is no input.

This is a brainless 1:1 replacement of OIS API and events with SDL API and OgreBites input events. I preserved our (very outdated) DearIMGUI integration. All our (quite convoluted) input processing stayed mostly intact. Only mouse/keyboard input is handled, joysticks + controllers + forcefeedback are temporarily disabled.

Status: build and runs, keyboard+mouse working, window is somehow oversized but responsive.

Codechanges:
* creating render window via OgreBites::ApplicationContext instead of OGRE Root directly - needed to receive events
* overriding OgreBites::ApplicationContext::createRoot() with our pre-existing setup code, to initialize `mRoot` as expected.
* calling `OgreBites::ApplicationContext::addInputListener()` to receive events.

STATUS:
- ogre imgui port: keyboard/mouse input working!
- ogre imgui port: FIXME: hacked Renderdash to build, but imgui will likely render into it - how to disable??
- ogre imgui port: builds and runs with default font.
- under Debug I get asserts about NewFrame()/EndFrame() mismatches, to be fixed later.

CODECHANGES:
* imgui deprecated - renamed ImGui::AlignFirstTextHeightToWidgets() to ImGui::AlignTextToFramePadding()
* imgui deprecated - ImGui::GetContentRegionAvailWidth() -> use ImGui::GetContentRegionAvail().x
* imgui deprecated -  GetWindowContentRegionWidth() function. -> 'GetWindowContentRegionMax().x
* imgui deprecated - SetScrollHere() -> SetScrollHereX() + SetScollHereY()
* imgui deprecated - ImGui::TreeAdvanceToLabelPos()      -> use ImGui::SetCursorPosX(ImGui::GetCursorPosX() + ImGui::GetTreeNodeToLabelSpacing());
* imgui deprecated - renamed SetNextTreeNodeOpen() to SetNextItemOpen().
* imgui deprecated - renamed ListBoxHeader(const char* label, ImVec2 size) to BeginListBox().
* imgui deprecated - removed BeginPopupContextWindow(const char*, int mouse_button, bool also_over_items) in favor of BeginPopupContextWindow(const char*, ImGuiPopupFlags flags) with ImGuiPopupFlags_NoOverItems.
* imgui deprecated - removed CalcListClipping() function. Prefer using ImGuiListClipper which can return non-contiguous ranges.
* imgui deprecated - ImGuiCol_ChildWindowBg              -> use ImGuiCol_ChildBg
* imgui deprecated - style: renamed ImGuiCol_ModalWindowDarkening to ImGuiCol_ModalWindowDimBg
* imgui deprecated - GetItemsLineHeightWithSpacing()     -> use GetFrameHeightWithSpacing()
* imgui deprecated - InputText: Renamed  ImGuiTextEditCallbackData to ImGuiInputTextCallbackData
* imgui deprecated - SetNextWindowContentWidth(float w)    ->    SetNextWindowContentSize(ImVec2(w, 0.0f))
* imgui deprecated - SetNextWindowPosCenter(ImGuiCond c=0) { SetNextWindowPos(GetMainViewport()->GetCenter(), c, ImVec2(0.5f,0.5f));
* ogre imgui port: use public `GetFont()/GetFontSize()` instead of internal `ImGuiContext->Font/FontSize`
* VehicleAI panel: `Push/PopItemFlag(Disabled)` (imgui_internal) -> `Begin/EndDisabled()` (public BETA API)

…of screen.

Deleted the windows-specific code setting ["monitorIndex"]- apparently OGRE does it by itself in `ApplicationContextSDL::createWindow()`, see OgreApplicationContextSDL.cpp line 56.
Original commit: 128a8e2 - "Attempt to fix the 'Rendering Device' setting" - Ulteq, 4 years ago (29.12.2018 13:53:57)

Also deleted setting ["windowProc"] - doesn't seem to have any effect, perhaps an OIS leftover.

Codechanges:
* GUIUtils, InputEngine: `getModifierKeyName()` replaced OIS keycodes with OgreBites/SDL keycodes
* ImGuiAngelscript: commented out some dead items, added (ImGuiKey) casts where needed, see ocornut/imgui#4921
* GUI_VehicleButtons: build fix for `ImGui::ImageButton()` - added parameter ID-string.
* GUI_RepositorySelector: updated thumbnail downloading to use new OGRE WorkQueue API, see https://github.com/OGRECave/ogre/blob/master/Docs/14-Notes.md#task-based-workqueue
* Application.h, main.cpp: added ✉️ `MSG_NET_DOWNLOAD_REPOTHUMB_SUCCESS`, used instead of OGRE WorkQueue's `addMainThreadTask` for consistency across the various background threads in the codebase.
* GodRaysManager: build fix - RENDER_QUEUE_9 was removed from OGRE, using 8.
* RttManager: rendersystem-specific `getProjectionMatrixRS()` was removed from OGRE, using portable  `getProjectionMatrix()`