st: Allow accelerated CopyTexImage from RGBA to RGB.

st: Allow accelerated CopyTexImage from RGBA to RGB.

There's nothing to worry about here -- the A channel just gets dropped by
the blit.  This avoids a segfault in the fallback path when copying from a
RGBA16_SINT renderbuffer to a RGB16_SINT destination represented by an
RGBA16_SINT texture (the fallback path tries to get/fetch to float
buffers, but the float pack/unpack functions are NULL for SINT/UINT).

Fixes KHR-GLES3.packed_pixels.pbo_rectangle.rgba16i on VC5.

v2: Extract the logic to a helper function and explain what's going on
    better.
v3: const-qualify args

Reviewed-by: Brian Paul <brianp@vmware.com>