Updated GLSL uniform/sampler handling from gallium-0.1 branch

Updated GLSL uniform/sampler handling from gallium-0.1 branch

Previously, the shader linker combined the uniforms used by the vertex and
fragment shaders into a combined set of uniforms.  This made the implementation
of glUniform*() simple, but was rather inefficient otherwise.  Now each shader
gets its own set of uniforms (no more modelview matrix showing up in the
fragment shader uniforms, for example).

cherry-picked by hand from gallium-0.1 branch