BGE Shaders: Make sure to parse only used uniforms

This fixes a problem that can occur when a shader source has been setup, and then
disabled on a custom shader that is in use.
BGE Shader UI: Fixing up the panel disabling a bit

The panels that were disabled for custom shaders were really for the
fragment shader. Now those panels (e.g. diffuse, specular) are only
disabled when a custom fragment shader is used, not when a custom
vertex shader is used.