mesa: Trim down PixelStorei implementation

Move _mesa_error call for INVALID_VALUE to one place.
Remove checks for previous value matching -- this was important when we
were flushing vertices before the update, but that hasn't happened for a
long time now.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
mesa: Make CompressedTexSubImage errors more consistent

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
mesa: Add extension enable for ARB_compressed_texture_pixel_storage

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
glapi: Add XML infrastructure for ARB_compressed_texture_pixel_storage

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
tests: Add new enum strings for ARB_compressed_texture_pixel_storage

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
mesa: Add new pixel pack/unpack state for
ARB_compressed_texture_pixel_storage

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
mesa: Emit errors for inconsistent compressed pixel store state

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
mesa: Extract computation of compressed pixel store params

This logic is reusable across CompressedTex*Image* and
GetCompressedTexImage; the strides calculated will also be needed
in the PBO validation functions to ensure that the referenced range of
bytes is valid.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
mesa: Compute proper strides for compressed texture pixel storage.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
mesa: Add pixel storage support for GetCompressedTexImage

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
docs: Mark off ARB_compressed_texture_pixel_storage

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>