st/mesa: fix texture image resource selection in st_render_texture()

st/mesa: fix texture image resource selection in st_render_texture()

If we're rendering to an incomplete/inconsistent (cube) texture, the
different faces/levels of the texture may be stored in different
resources.  Before, we always used the texture object resource.  Now,
we use the texture image resource.  In normal circumstances, that's
the same resource.  But in some cases, such as the Piglit
fbo-incomplete-texture-03 test, the cube faces are in different
resources and we need to render to the texture image resource.

Fixes fbo-incomplete-texture-03 with VMware driver.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>