st/mesa: fix glCopyTexSubImage crash
st/mesa: fix glCopyTexSubImage crash

Fixes a WebGL crash.  The dest texture image is at level 2 and is of
size 1x1 texel.  The st texture image is a stand-alone resource, not
a pointer into a complete mipmap.  So the resource has one level and
trying to write to level 2 blows up.

Fixes http://bugs.freedesktop.org/show_bug.cgi?id=53314
and http://bugs.freedesktop.org/show_bug.cgi?id=53319

Note: This is a candidate for the 8.0 branch.

Reviewed-by: Jose Fonseca <61b5a29db2c0650f7c8510f93eece059c1b09320@vmware.com>

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