swrast: fix broken _swrast_depth_clamp_span()
swrast: fix broken _swrast_depth_clamp_span()

The integer Z clamping range depends on the number of bits
in the Z buffer because that's the scale factor used when we
transform NDC coords by the viewport/depth range.

Fixes fd.o bug #25972 but only for Z buffers up to a depth
of 30 bits.  Beyond that we get into messy integer overflow
issues and things fall apart.

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