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/r_things.cpp

@@ -2500,7 +2500,7 @@ void R_DrawParticle (vissprite_t *vis)
 		{
 			DWORD bg = bg2rgb[*dest];
 			bg = (fg+bg) | 0x1f07c1f;
-			*dest++ = RGB32k[0][0][bg & (bg>>15)];
+			*dest++ = RGB32k.All[bg & (bg>>15)];
 		} while (--count);
 		dest += spacing;
 	} while (--ycount);