copyteximage: Use Driver's AllocTextureImageBuffer instead of TexImage

copyteximage: Use Driver's AllocTextureImageBuffer instead of TexImage

Call Driver.AllocTextureImageBuffer rather than calling
Driver.TexImage with NULL data, format=GL_NONE and type=GL_NONE.

This avoids setting ctx->Unpack, which can lead to incorrectly
trying to upload data.

The GLES3 GTF program's packed_pixels_pbo test was triggering
an error for i965 with the previous code.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>