util: Include RGTC formats in the list of block formats for rgbw_texture.

No tests successfully use rgbw and RGTC yet, since we can't render to
rgtc.  But when we go write some, this should reduce surprises.

tests/util/piglit-util-gl.c

@@ -905,6 +905,10 @@ piglit_rgbw_texture(GLenum format, int w, int h, GLboolean mip,
 				case GL_COMPRESSED_RGBA_S3TC_DXT5_EXT:
 				case GL_COMPRESSED_RGB_FXT1_3DFX:
 				case GL_COMPRESSED_RGBA_FXT1_3DFX:
+				case GL_COMPRESSED_RED_RGTC1:
+				case GL_COMPRESSED_SIGNED_RED_RGTC1:
+				case GL_COMPRESSED_RG_RGTC2:
+				case GL_COMPRESSED_SIGNED_RG_RGTC2:
 					if (size == 4)
 						color = red;
 					else if (size == 2)