draw: initialize shader inputs
draw: initialize shader inputs

This should make the code more robust if a shader tries to use inputs which
aren't defined by the vertex element layout (which usually shouldn't happen).

No piglit change.

Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler