mesa: Add _mesa_format_fallback_rgbx_to_rgba() [v2]

mesa: Add _mesa_format_fallback_rgbx_to_rgba() [v2]

The new function takes a mesa_format and, if the format is an alpha
format with a non-alpha variant, returns the non-alpha format.
Otherwise, it returns the original format.

Example:
  input -> output

  // Fallback exists
  MESA_FORMAT_R8G8B8X8_UNORM -> MESA_FORMAT_R8G8B8A8_UNORM
  MESA_FORMAT_RGBX_UNORM16 -> MESA_FORMAT_RGBA_UNORM16

  // No fallback
  MESA_FORMAT_R8G8B8A8_UNORM -> MESA_FORMAT_R8G8B8A8_UNORM
  MESA_FORMAT_Z_FLOAT32 -> MESA_FORMAT_Z_FLOAT32

i965 will use this for EGLImages and DRIimages.

v2 (Jason Ekstrand):
 - Use mako
 - Rework to be easier to read
 - Write directly to the output file

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