LibGL: Add capabilities to context parameters

Context parameters are LibGL's way of centrally defining all parameters
that can be retrieved through `glGetBoolean*`, `glGetInteger*`, etc.
The spec describes that capabilities such as `GL_LIGHTING` can also be
retrieved through these methods, but it should not be possible to
retrieve random boolean parameters through `glIsEnabled`. For example,
`GL_UNPACK_LSB_FIRST` can only be retrieved through `glGet*`.

This moves reading of capabilities to `::get_context_parameter` and
implements its use in `::gl_is_enabled`.

Userland/Libraries/LibGL/SoftwareGLContext.h

@@ -28,6 +28,7 @@ namespace GL {
 
 struct ContextParameter {
     GLenum type;
+    bool is_capability { false };
     u8 count { 1 };
     union {
         bool boolean_value;