Improved multisampling support on iOS and Android

On Android, some extensions are not accessible using DLLImport and we need to use eglGetProcAddress instead. The NVIDIA Shield (1st Gen and Tablet) and a few other devices depending on GPU and driver version can encounter this issue.

This PR fixes the problem by using eglGetProcAddress when retrieving extensions on Android platforms. It also should behave better in case of an issue retrieving the function pointers, even if the driver claims the extension is supported.

Tested on Nexus 9, Galaxy Tab 4, Galaxy Tab 2 7.0, NVIDIA Shield (Portable), iPhone 6, iPad Mini