Hull heating shader effect

# Description:
Apply a simple heating shader effect based on the hull temperature and direction of travel.

It adds one texture which controls the heat gradient coloring. If you want to make it fade in later than squash the gradient further up and extand the black section towards the middle. Vice versa with the white end if you want it to ramp up earlier.

@shadmar you're a shader Ninja :) any advice on the shader itself?

# Testing:
The best way to test is to launch with a powerful ship like the PumpkinSeed and make sure it hasn't got any atmospheric shielding. It's pretty clear when it's getting toasty!

Andy

@robn / @johnbartholomew - this is no longer WIP so is ready for review whenever there's opportunity. Check it out as it's quite a nice effect I think :) Wish I'd done it sooner!