rusticl: reuse PipeContext

Gallium drivers are likely to leak CPU and GPU resources as with OpenGL
they rarely have to create more than a single pipe_context.

This also reduces the cost of creating CL queues.

In order to debug drivers leaking memory a new debug option is added to
disable the reuse of PipeContexts

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30888>

docs/envvars.rst

@@ -1153,6 +1153,7 @@ Rusticl environment variables
 
    - ``allow_invalid_spirv`` disables validation of any input SPIR-V
    - ``clc`` dumps all OpenCL C source being compiled
+   - ``no_reuse_context`` pipe_contexts are not recycled
    - ``perf`` prints a warning when hitting slow paths once
    - ``perfspam`` same as perf, but doesn't skip same warnings
    - ``program`` dumps compilation logs to stderr