[iOS] Maintain compatibility with Xamarin.iOS

Compatibility tested with apitest.
[iOS] Maintain compatibility with Xamarin.iOS

Compatibility tested with apitest.

Remaining differences (ref -> out parameters):
- GetBoolean
- GetBufferParameter
- GetFloat
- GetFramebufferAttachmentParameter
- GetInteger
- GetProgram
- GetRenderbufferParameter
- GetShader
- GetShaderPrecisionFormat
- GetTexParameter
- GetVertexAttrib