Expose setting to configure hardware acceleration policy

Accelerated compositing mode is required to achieve acceptable performance on certain hardware that we care about, but also causes unacceptable performance degredation on other hardware. It also significantly increases memory usage requirements.

Let's allow the user to choose. Distributors can use a gsettings override if needed to target particular hardware.

This merge request changes the default policy to on-demand, but I might change it back depending on how it goes.