fix SKYDEFS scroll speed (#2135)

* fix SKYDEFS scroll speed

Fixes #2107

* add scrolling sky interpolation

* add `scrolly` interpolation, check fo pause/menu

Co-Authored-By: @rfomin

Author: fabiangreffrath

src/r_plane.c

@@ -39,6 +39,7 @@
 #include "doomstat.h"
 #include "doomtype.h"
 #include "i_system.h"
+#include "i_video.h"
 #include "m_fixed.h"
 #include "r_bmaps.h" // [crispy] R_BrightmapForTexName()
 #include "r_data.h"
@@ -413,9 +414,21 @@ static void DrawSkyTex(visplane_t *pl, skytex_t *skytex)
     dc_texheight = textureheight[texture] >> FRACBITS;
     dc_iscale = FixedMul(skyiscale, skytex->scaley);
 
-    dc_texturemid += skytex->curry;
+    fixed_t deltax, deltay;
+    if (uncapped && leveltime > oldleveltime)
+    {
+        deltax = LerpFixed(skytex->prevx, skytex->currx);
+        deltay = LerpFixed(skytex->prevy, skytex->curry);
+    }
+    else
+    {
+        deltax = skytex->currx;
+        deltay = skytex->curry;
+    }
+
+    dc_texturemid += deltay;
 
-    angle_t an = viewangle + FixedToAngle(skytex->currx);
+    angle_t an = viewangle + (deltax << (ANGLETOSKYSHIFT - FRACBITS));
 
     for (int x = pl->minx; x <= pl->maxx; x++)
     {

src/r_sky.c

@@ -176,12 +176,16 @@ void R_UpdateSky(void)
     }
 
     skytex_t *background = &sky->skytex;
+    background->prevx = background->currx;
+    background->prevy = background->curry;
     background->currx += background->scrollx;
     background->curry += background->scrolly;
 
     if (sky->type == SkyType_WithForeground)
     {
         skytex_t *foreground = &sky->foreground;
+        foreground->prevx = foreground->currx;
+        foreground->prevy = foreground->curry;
         foreground->currx += foreground->scrollx;
         foreground->curry += foreground->scrolly;
     }

src/r_skydefs.h

@@ -37,8 +37,10 @@ typedef struct
     double mid;
     fixed_t scrollx;
     fixed_t currx;
+    fixed_t prevx;
     fixed_t scrolly;
     fixed_t curry;
+    fixed_t prevy;
     fixed_t scalex;
     fixed_t scaley;
 } skytex_t;