Generalize EGL support on windows

[dliepelt]

Currently the EGL support on Windows expects the EGL/Angle platform, however there are other EGL implementations available like Imagination Technologies' PVRVFrame. This change adds a compile-time check for EGL/Angle, if unavailable a more generic way for getting the EGL display handle is chosen.

This PR also contains a fix for the GraphicsWindowWin32 OpenGL context creation when using EGL which only worked if GLES3 was available.