st/mesa: fix glDrawPixels(GL_DEPTH_COMPONENT) regression

Commit 1454f20a991ddda35f1a2ffda953012078b407ba caused the regression.
When the vertex shader emitted both a texcoord and color we were grabbing
the wrong vertex attributes.  Fix the draw_quad() code to put texcoords
in slot[1] and color in slot[2].  That's a bit cleaner than changing
the vertex shader code.

Tested with progs/tests/zreaddraw.c
1 file changed