Beta upgrades: optimised performance

As the framerate was plunging towards the dreaded 15s, I made some
upgrades.

A sizeable hit was coming from the HUD elements, most likely the text
rendering. I've replaced the text with static images for an immediate
gain. The HUD was generally upgraded into its own class.

Rendering pipeline optimised. Instead of using two renderers, I now use
just one, with a few extra options to get the HUD and viewscreen
distortion separate from the "real" elements. Additional optimisations
and corrections made to the lighting calculations.

Main program now automatically assumes a 16:9 display that fits the
screen.

Main program can now toggle between camera and mouse control, if camera
is present.

Bloom shader is now nicer and not nearly so fiddly to work with. It uses
both the maximum detected colour, and the sum of all sampled colours, to
create nice light bloom.