mir: Support creation of EGLImages from gbm buffer objects

Support creation of EGLImages from gbm buffer objects, which we consider
the native pixmap type (EGL_NATIVE_PIXMAP_KHR) of the Mir EGL platform.

Co-authored-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
gbm: Allow writing to gbm_bos that have a DRI Image
gbm: Update DRI2_LOADER version
Merge pull request #5 from afrantzis/egl-platform-mir-egl-image

Egl platform mir egl image