Clear out pending window operations after start

[Jacalz]

Description:

Found a small memory leak when looking through the code. Let's make sure to clear out the pending operations once when have executed them. This is hard to test with how glfw tests works so did not figure a way to add a test.

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[andydotxyz]

Thanks :)