Use SDL_Color for colors instead of colourTransform

colourTransform is a struct with only one member, a Uint32. The issue
with `Uint32`s is that it requires a bunch of bit shifting logic to edit
the colors. The issue with bit shifting logic is that people have a
tendency to hardcode the shift amounts instead of using the shift amount
variables of the SDL_PixelFormat, which makes it annoying to change the
color masks of surfaces.

This commit fixes both issues by unhardcoding the bit shift amounts in
DrawPixel and ReadPixel, and by axing the `Uint32`s in favor of using
SDL_Color.

According to the SDL_PixelFormat documentation (
https://wiki.libsdl.org/SDL2/SDL_PixelFormat ), the logic to read and
draw to pixels from colors below 32-bit was just wrong. Specifically,
for 8-bit, there's a color palette used instead of some intrinsic color
information stored in the pixel itself. But we shouldn't need that logic
anyways because we don't use colors below 32-bit. So I axed that too.

desktop_version/src/CustomLevels.h

@@ -155,8 +155,8 @@ public:
 
     int version;
 
-    Uint32 getonewaycol(const int rx, const int ry);
-    Uint32 getonewaycol(void);
+    SDL_Color getonewaycol(int rx, int ry);
+    SDL_Color getonewaycol(void);
     bool onewaycol_override;
 };