pan/decode: Use GLSL style formats/swizzles

pan/decode: Use GLSL style formats/swizzles

This dramatically reduces visual clutter: now an entire
attribute/varying record looks something like:

    rgba32f attribute_0[16].bgra;

which is equivalent to the raw structure:

{
   .index = 0,
   .format = MALI_FORMAT_RGBA32F,
   .swizzle = (MALI_CHANNEL_BLUE << 9) | ....,
   .src_offset = 16,
}

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>