Use half float instead of int16 for color buffer

This allows us to use values greater than 1 in the color buffer and as
the color output of a fragment shader, so we don't have to worry so much
about values getting clamped in multi-stage shaders.

src/engine/renderer/tr_image.cpp

@@ -2593,7 +2593,7 @@ static void R_CreateCurrentRenderImage()
 		}
 		else
 		{
-			imageParams.bits |= IF_RGBA16;
+			imageParams.bits |= IF_RGBA16F;
 		}
 	}