mesa: do depth/stencil format conversion in glGetTexImage
mesa: do depth/stencil format conversion in glGetTexImage

glGetTexImage(GL_DEPTH_STENCIL, GL_UNSIGNED_INT_24_8) was just
using memcpy() instead of _mesa_unpack_uint_24_8_depth_stencil_row()
to convert texels from the hardware format to the GL format.

Fixes issue reported by David Meng at Intel.  The new piglit
ext_packed_depth_stencil-getteximage test checks for this bug.

Also, add some format/type assertions.  We don't yet handle the
GL_FLOAT_32_UNSIGNED_INT_24_8_REV type.  That should be fixed in
a follow-on patch.

Reviewed-by: Eric Anholt <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@anholt.net>
Cc: "10.0" "10.1" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>

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