vc4: Don't throw out the index offset in the shadow index buffer path.
vc4: Don't throw out the index offset in the shadow index buffer path.

When we upload shadow indices at draw time, we need the source offset.
Fixes the piglit draw-elements test.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler