Compile terrain shader program earlier when terrain is detected

[karimnaaji]

mapbox-gl-js version: 2.0.0

Following preemptive shader compilation (a30ccc8) with terrain, we noticed that we have a lot of "cache miss" once preemptive compilation has done its first pass:

This is due to the highlighted line, rtt is only set after a state change in the terrain system, which then inject RENDER_TO_TEXTURE to a few variant of our shaders.

A fix for that would be to immediately add this define to the program definition whenever terrain is detected by default in the style, this would save a lot of shader variant being compiled with delay. Terrain shader variant could benefit from the preemptive shader compilation strategy already introduced.

Eventually reevaluate KHR_parallel_shader_compile.