backends: Ensure we are always current when creating dmabufs
It's necessary so that GL does the right thing, therefore instead of always calling it before importDmaBufAsImage, we just do it within. This is important because otherwise we might forget like we were on the WaylandBackend. The patch looks a bit bigger because we neeeded to make makeCurrent const, which we do here.
Edited by Aleix Pol Gonzalez