Add Raspberry PI to CMake logic (use EGLContext)

[headupinclouds]

The current Android EGL context should support generic EGL (I see no Android specific code). Need to confirm/test. If so, the CMake logic can be adapted to make this available for any EGL platform.

[headupinclouds]

renamed GLContextAndroid -> EGLContext in #9
CMake logic + testing is still needed for raspberry pi