Add Raspberry Pi GLES context init code

SDL/OpenGL(GLES)

Add the init code for EGL context initialization so GLES1 is used on the Raspberry Pi for final rendering.
The class OpenGLRPIGraphicsManager was added. It's located in backends/graphics/glesrpi, and is instantiated from backends/platform/sdl/sdl.cpp in OSystem_SDL::initBackend.
Raspberry Pi is autodetected at build time in the configure script.