Beta upgrades: fighters, background

New enemy ship added: missile corvettes or "fighters". Fast, small, and
fond of rapid-fire homing missiles. Their main purpose is to add visual
interest to the passive mode, but they're fun enemies. They don't spawn
in the initial wave, but form ~50% of subsequent spawns.

Background revised to be less intrusive, and display better on high-res
displays. This means it's now 3840*2160 and forms a substantial part of
the entire project.

Frame rate now capped at 60 to take advantage of performance-boosting
options. Simulation tick is capped at 2.0, meaning it will cope with a
framerate as low as 30 before it starts slowing down time to ensure
smoothness.

New key command: press 'B' or 'b' to toggle bloom. This can result in a
substantial frame rate increase.

Plasma bullets now charge faster.

New fragment shader: tex_deferCompositor_noChromAb.frag.glsl. This
performs the function of the normal compositor shader, but without
splitting the warp cursor into three colour channels to perform
chromatic aberration. This was tested and found to have no measurable
effect on frame rate, so it was taken out again to preserve quality.