mesa: Add fallback from RGB_FLOAT16 to RGBA_FLOAT16 before RGBA_FLOAT32.

Not all i965 hardware can do RGB float16, and this will at least save
half the memory and have expected behavior in terms of precision.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/main/texformat.c

@@ -356,6 +356,7 @@ _mesa_choose_tex_format( struct gl_context *ctx, GLint internalFormat,
 	    break;
          case GL_RGB16F_ARB:
 	    RETURN_IF_SUPPORTED(MESA_FORMAT_RGB_FLOAT16);
+	    RETURN_IF_SUPPORTED(MESA_FORMAT_RGBA_FLOAT16);
 	    RETURN_IF_SUPPORTED(MESA_FORMAT_RGB_FLOAT32);
 	    RETURN_IF_SUPPORTED(MESA_FORMAT_RGBA_FLOAT32);
 	    break;