Enhancements:
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* several rendering speed improvements #2925

Bug fixes:
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* add regression test for #2912       #2914
* fix a file handle not being closed  #2915
* fix an incorrect syntax RuntimeError  #2920
* fix custom compiler languages not being possible  #2927
* fix OS vars not correctly respecting test prefix; add test  #2932
* fix incorrect python versions showing up in test phase paths (SP_DIR)  #2932
* fix test/files functionality for outputs; add test  #2934

Contributors:
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* @mbargull
* @msarahan
* @nicoddemus
* @rainwoodman
* @sodre
* @tomashek