compiler: Save a single copy of the softfp64 shader in the context.

We were recompiling the softfp64 library of functions from GLSL to NIR every time we compiled a shader that used fp64. Worse, we were ralloc stealing it to the GL context. This meant that we'd accumulate lots of copies for the lifetime of the context, which was a big space leak.

Instead, we can simply stash a single copy in the GL context, and use it for subsequent compiles. Having a single copy should be fine from a memory context point of view: nir_inline_function_impl already clones the necessary nir_function_impl's as it inlines.

KHR-GL45.enhanced_layouts.ssb_member_align_non_power_of_2 was previously OOM'ing a system with 16GB of RAM when using softfp64. Now it finishes much more quickly and uses only ~200MB of RAM.