mesa: Introduce a _mesa_format_has_color_component() helper.

When considering color write masks, we often want to know whether an
RGBA component actually contains any meaningful data.  This function
provides an easy way to answer that question, and handles luminance,
intensity, and alpha formats correctly.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Tested-by: Dylan Baker <baker.dylan.c@gmail.com>

src/mesa/main/formats.c

@@ -2206,6 +2206,35 @@ _mesa_format_num_components(mesa_format format)
 }
 
 
+/**
+ * Returns true if a color format has data stored in the R/G/B/A channels,
+ * given an index from 0 to 3.
+ */
+bool
+_mesa_format_has_color_component(mesa_format format, int component)
+{
+   const struct gl_format_info *info = _mesa_get_format_info(format);
+
+   assert(info->BaseFormat != GL_DEPTH_COMPONENT &&
+          info->BaseFormat != GL_DEPTH_STENCIL &&
+          info->BaseFormat != GL_STENCIL_INDEX);
+
+   switch (component) {
+   case 0:
+      return (info->RedBits + info->IntensityBits + info->LuminanceBits) > 0;
+   case 1:
+      return (info->GreenBits + info->IntensityBits + info->LuminanceBits) > 0;
+   case 2:
+      return (info->BlueBits + info->IntensityBits + info->LuminanceBits) > 0;
+   case 3:
+      return (info->AlphaBits + info->IntensityBits) > 0;
+   default:
+      assert(!"Invalid color component: must be 0..3");
+      return false;
+   }
+}
+
+
 /**
  * Return number of bytes needed to store an image of the given size
  * in the given format.