mesa: fix return value when clipping {Read,Draw}Pixels height <= 0
mesa: fix return value when clipping {Read,Draw}Pixels height <= 0

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

(cherry picked from master, commit 7aeaca33c331f70d507fc83583b13b8d9fc3e847)

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