Change RGB32k to a union of BYTE[32][32][32] and BYTE[32*32*32]

- Clang's optional runtime array bounds checking doesn't understand when we
  intentionally "overflow" by doing this:
    RGB32k[0][0][colorval]
  It will warn that it was accessed at an index will past the bounds
  of type 'BYTE [32]', which makes it less than useful for catching real
  array bounds overflows. So now do this:
    RGB32k.All[colorval]
  And if you want this:
    RGB32k[r][g][b]
  Now do this:
    RGB32k.RGB[r][g][b]

src/textures/multipatchtexture.cpp

@@ -534,7 +534,7 @@ void FMultiPatchTexture::MakeTexture ()
 			{
 				if (*out == 0 && in[3] != 0)
 				{
-					*out = RGB32k[in[2]>>3][in[1]>>3][in[0]>>3];
+					*out = RGB32k.RGB[in[2]>>3][in[1]>>3][in[0]>>3];
 				}
 				out += Height;
 				in += 4;