st/mesa: fix bitmap,drawpix,drawtex for PIPE_CAP_TGSI_TEXCOORD

NOTE: Changed the semantic index for the drawtex coordinate to
be the texture unit index instead of always 0.
Not sure if this is correct but since the value seems to depend
on the unit it would make sense to use different varying slots.
nv50,nvc0: disable DEPTH_RANGE_NEAR/FAR clipping during blit

We send position.z == 0, DEPTH_RANGE may be some arbitrary range
not including 0 (for exmaple in piglit's hiz tests).
nvc0: fix for 2d engine R source formats writing RRR1 and not R001
nvc0: honour scaled coordiantes setting for linear textures
nv50: account for pesky prefetch in size calculation of linear textures
nvc0: place staging textures in GART and map them directly