draw: cliptest and viewport done in a single loop in vertex shader

draw: cliptest and viewport done in a single loop in vertex shader

Cliptesting now done at the end of vs in draw_llvm instead of
draw_pt_post_vs.

Added viewport mapping transformation and further cliptesting to
vertex shader in draw_llvm.c

Alternative path where vertex header setup, clip coordinates store,
cliptesting and viewport mapping are done earlier in the vertex
shader.

Still need to hook this up properly according to the return value of
"draw_llvm_shader" function.