when mirroring the texture corrdinates the indices must be mirrored as
well and the half pixel shift must be applied in reverse.
Fixes a number of tests from:
dEQP-GLES31.functional.texture.gather.offset.*
dEQP-GLES31.functional.texture.gather.offsets.*
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>