Add QGL_GetDriverInfo for systems without driver override

A cleaned-up version of https://github.com/viciious/qfusion/commit/7886b80fb6cac3b5d5c0a7112e29d62a3293227f

QGL now specifies which convar to use to override the default GL driver DLL and whether to use it at all.

This solves the issue with updating to Android 4.3: if the engine ran on a GLES3-supporting device on an old OS version before the update, `libGLESv2.so` would be stored in `config.cfg`, which is not the valid library for GLES3.