st/vdpau: Avoid constness cast warnings.
st/vdpau: Avoid constness cast warnings.

Fixes MSVC

  warning C4090: '=' : different 'const' qualifiers

Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>
Reviewed-by: Christian König <c7ea837d7a46effe4232b086213468b8b31643bf@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler