WIP: more clear fixes

Check this.. there still seems like there might be an issue with stencil
buffer getting cleared when we are trying to only clear depth.
WIP size optimization is not right when start!=0..

And probably for index'd drawing too.  There are MIN/MAX vtx registers,
which we could possible set to get the same benefit (ie. don't prefetch
unneeded data from VBO)
WIP: I think I need PROVOKING_VTX_LAST set for these
WIP hack to convert quads to trilist w/ index buffer

This won't correctly handle the case where we already have an index
buffer.  And probably could be made more generalized and put in gallium
core or helper..

but for now this helps GL1.4 stuff (like gnome-shell) work.
WIP track maximal scissor bounds

Optimize out parts of the render target that are scissored out.