xwayland/glamor/gbm: Use EGL_NO_CONTEXT with EGL_NATIVE_PIXMAP_KHR

This is required per https://www.khronos.org/registry/EGL/extensions/KHR/EGL_KHR_image_pixmap.txt .

Mesa hasn't enforced it, but it will soon: mesa/mesa!454 (merged)

Fixes: 2f113d68 ("xwayland: Add glamor and DRI3 support")

Merge request reports