draw: fix point/line/tri flushing bug in vbuf code
draw: fix point/line/tri flushing bug in vbuf code

Need to reset the point/line/tri functions to point to the "first"
versions whenever we flush vertices.  Fixes unfilled polygon rendering
errors seen in demos/samples/logo.c.  See comments for more info.

NOTE: This is a candidate for the 7.10 branch.

benaadams/glsl-optimizer,dellis1972/glsl-optimizer,bkaradzic/glsl-optimizer,mapbox/glsl-optimizer,dellis1972/glsl-optimizer,djreep81/glsl-optimizer,adobe/glsl2agal,zeux/glsl-optimizer,mapbox/glsl-optimizer,adobe/glsl2agal,tokyovigilante/glsl-optimizer,djreep81/glsl-optimizer,metora/MesaGLSLCompiler,KTXSoftware/glsl2agal,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,zz85/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,tokyovigilante/glsl-optimizer,tokyovigilante/glsl-optimizer,djreep81/glsl-optimizer,KTXSoftware/glsl2agal,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,KTXSoftware/glsl2agal,KTXSoftware/glsl2agal,mcanthony/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,wolf96/glsl-optimizer,bkaradzic/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,dellis1972/glsl-optimizer,zeux/glsl-optimizer,zeux/glsl-optimizer,zeux/glsl-optimizer,jbarczak/glsl-optimizer,mcanthony/glsl-optimizer,adobe/glsl2agal,benaadams/glsl-optimizer,tokyovigilante/glsl-optimizer,wolf96/glsl-optimizer,tokyovigilante/glsl-optimizer,wolf96/glsl-optimizer,zz85/glsl-optimizer,zz85/glsl-optimizer,zz85/glsl-optimizer,metora/MesaGLSLCompiler,mapbox/glsl-optimizer,adobe/glsl2agal,mapbox/glsl-optimizer,benaadams/glsl-optimizer,zz85/glsl-optimizer,wolf96/glsl-optimizer,jbarczak/glsl-optimizer,bkaradzic/glsl-optimizer,djreep81/glsl-optimizer,KTXSoftware/glsl2agal,metora/MesaGLSLCompiler,jbarczak/glsl-optimizer,dellis1972/glsl-optimizer,bkaradzic/glsl-optimizer,djreep81/glsl-optimizer,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,mapbox/glsl-optimizer,adobe/glsl2agal