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.cpp

@@ -1604,7 +1604,7 @@ void customlevelclass::generatecustomminimap(void)
 
 // Return a graphics-ready color based off of the given tileset and tilecol
 // Much kudos to Dav999 for saving me a lot of work, because I stole these colors from const.lua in Ved! -Info Teddy
-Uint32 customlevelclass::getonewaycol(const int rx, const int ry)
+SDL_Color customlevelclass::getonewaycol(const int rx, const int ry)
 {
     const RoomProperty* const room = getroomprop(rx, ry);
     switch (room->tileset) {
@@ -1763,7 +1763,7 @@ Uint32 customlevelclass::getonewaycol(const int rx, const int ry)
 }
 
 // This version detects the room automatically
-Uint32 customlevelclass::getonewaycol(void)
+SDL_Color customlevelclass::getonewaycol(void)
 {
 #ifndef NO_EDITOR
     if (game.gamestate == EDITORMODE)