Implement most CSFML 2.6.0 features

[Marioalexsan]

This PR aims to implement most things from CSFML 2.6.0, along with some stuff that might be missing from CSFML 2.5.2.

Change list from 2.6.0:

• sfFont_setSmooth / sfFont_isSmooth
• sfBlendMin & sfBlendMax
• sfFloatRect_getPosition/sfIntRect_getPosition & sfFloatRect_getSize/sfIntRect_getSize
• Document MP3 support
• Add directional arrow resize cursors
• sfFont_getBoldKerning
• Add sf::Vulkan functions (partial)
• Add scancode support (partial)
• sfRenderTexture_isSrgb / sfRenderWindow_isSrgb
• sfImage_saveToMemory with sfBuffer
• sf::WindowBase

Other things which were done in this PR (probably missing from CSFML 2.5.2?):

• drawVertexBufferRange implementations from CSFML
• TextureCoordinateType implementation
• sRGB creation functions
• sfContext_isExtensionAvailable and sfContext_getFunction
• sfWindowBase_createVulkanSurface
• some other stuff I probably missed

Features which were not done due to issues:

• sfVulkan_getGraphicsRequiredInstanceExtensions - needs a CSFML release containing CSFML#229
• sfKeyboard_getDescription - this function returns an owning C pointer (const char*). I'm not sure how to handle the deallocation for it since there's no corresponding function to free the string

By the looks of it, this PR might overlap / conflict with #236, #239, #240, and maybe #238.

This PR is a mess and I don't expect it to pass first inspection, so any feedback is welcome.

[Marioalexsan]

Rebased from master

[eXpl0it3r]

Some small var changes

[DemoXinMC]

Is there some benefit to wrapping WindowBase that I'm missing? It's not extensible from C# as it's basically used as an interface in SFML.

Also, I realize that var is pretty acceptable to use, but we're using explicit IntPtr throughout most of the wrapping IIRC. Does it make sense to switch to var right now on this PR?

[Marioalexsan]

WindowBase is not quite an interface from what I understand. It's supposed to be a very basic window so one can use OpenGL, Vulkan etc. on top of it without SFML intervening.

[eXpl0it3r]

Is there some benefit to wrapping WindowBase that I'm missing? It's not extensible from C# as it's basically used as an interface in SFML.

What do you mean with "wrapping". The point of WindowBase is to create a window without OpenGL context.
It's probably not as useful since it's tricky to use pure OpenGL or Vulkan in C#

Also, I realize that var is pretty acceptable to use, but we're using explicit IntPtr throughout most of the wrapping IIRC. Does it make sense to switch to var right now on this PR?

I wouldn't change it on this PR, it's already mixing a lot of things.

[eXpl0it3r]

LGTM

[eXpl0it3r]

Thank you for implementing this all! 🙂