drivers/meta: Accept GL_TEXTURE_3D as target for tex image decompression

drivers/meta: Accept GL_TEXTURE_3D as target for tex image decompression

An assert is currently raised, preventing decompression of a texture image into
a GL_TEXTURE_3D target. I have not found any spec wording that would explain
this, or implementation detail that would prevent it. And in any case, the
driver should not cause a crash upon user input arguments.

Fixes most failing subcases in CTS tests:
* GL44-CTS.gtf32.GL3Tests.packed_pixels.packed_pixels_pixelstore
* GL45-CTS.gtf32.GL3Tests.packed_pixels.packed_pixels_pixelstore

These tests were crashing the driver before. Now they just fail, but due
to an unrelated issue affecting 2 out of the 45 test subcases.

No regressions observed against piglit or CTS-GL.

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