gallium: fix memory corruption in u_gen_mipmap.c
gallium: fix memory corruption in u_gen_mipmap.c

Remove the old/initial vbuf allocation in util_create_gen_mipmap().
We were allocating a small vbuf at this point so get_next_slot() didn't have
as large of buffer as it expected.  So all but the first set_vertex_data()
was writing out of bounds.

Also added some comments.

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