qquickviewcomparison example crashes when using EGL and Mesa (Intel)

We are using surfaceless contexts for QOffscreenSurface on EGL , when available (new in 5.5), and so calling OpenGL functions relying on the draw/read framebuffer with FBO 0 bound is undefined behavior. (in case of Mesa glViewport is apparently such a function, at least when running on Intel)

This means anything using text rendering in a QQuickWidget may potentially crash when using EGL, depending on the drivers.