util: fix DXT1 RGBA texture compression if the source color is (0, 0, 0, 0)
util: fix DXT1 RGBA texture compression if the source color is (0, 0, 0, 0)

This is a workaround for a bug in libtxc_dxtn.

Fixes:
- piglit/GL_EXT_texture_compression_s3tc/fbo-generatemipmap-formats

Signed-off-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>

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