gallium: Introduce a new format to disambiguate PIPE_FORMAT_R8G8B8A8_UNORM.

PIPE_FORMAT_R8G8B8A8_UNORM now means just

  R8 G8 B8 A8

PIPE_FORMAT_R8G8B8A8_UNORM_REV means

  A8 B8 G8 R8

And the thumb rule is vertex buffers refer to PIPE_FORMAT_R8G8B8A8_UNORM,
while textures refer to PIPE_FORMAT_R8G8B8A8_UNORM_REV.

PIPE_FORMAT_R8G8B8A8_UNORM_REV is just a temporary name.

src/gallium/include/pipe/p_format.h

@@ -158,6 +158,8 @@ enum pipe_format {
    PIPE_FORMAT_DXT3_SRGBA            = 108,
    PIPE_FORMAT_DXT5_SRGBA            = 109,
 
+   PIPE_FORMAT_R8G8B8A8_UNORM_REV    = 110,
+
    PIPE_FORMAT_COUNT
 };