Cleanup: Spelling Mistakes

This patch fixes many minor spelling mistakes, all in comments or
console output. Mostly contractions like can't, won't, don't, its/it's,
etc.

Differential Revision: https://developer.blender.org/D11663

Reviewed by Harley Acheson

tests/gtests/CMakeLists.txt

@@ -1,6 +1,6 @@
 
 if(WITH_GTESTS)
-  # Otherwise we get warnings here that we cant fix in external projects
+  # Otherwise we get warnings here that we can't fix in external projects
   remove_strict_flags()
 
   # Build common test executable used by most tests