st/mesa: always call FLUSH_CURRENT before st->pipe->flush()
st/mesa: always call FLUSH_CURRENT before st->pipe->flush()

Undoes part of commit f455ca6 which would permit pipe->flush() to be
called while the VBO module still had its buffers mapped.  Some
drivers care deeply about this.  Sorry for not spotting this earlier.

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