st3m_gfx: change default graphics mode to 16bpp+osd+low_latency

This is a measure taken against issue #131 (closed) which seems to be a race/concurrent access issue when OSD is combined with having 3 concurrent ctx drawlists in flight.

For apps using the accelerometer or gyroscope to compute transforms/actions this is an improvement, this might also generally be a benefit for many music interfaces. In the cases where the deeper pipeline brings benefit apps can set a custom graphics mode with deeper pipeline and no osd, though for instance in the menus the frame rate achieved seems to now be the same for both types.