client: propose that MirBuffer is the EGLNativePixmapType for the mir platform. Include an example of buffer import to GL texture via the mir client api.
Also introduce the MIR_EXTENSION_ANDROID_EGL, with hooks for getting the EGLNativePixmapType and EGLNativeDisplayType. EGLNativeWindowType will be added after some MirRenderSurface improvements are proposed.
note: egldiamond will only import buffers on android, as the example shim subloads EGL_NATIVE_BUFFER_ANDROID. Other platforms have a sensible fallback.
Approved by Andreas Pokorny, mir-ci-bot, Chris Halse Rogers.