Use more precise return types for getimagesize* functions

[MidnightDesign]

• Key 0 (the image width) is always >= 0
• Key 1 (the image height) is always >= 0
• Key 2 is always one of the IMAGETYPE_* constants

[MidnightDesign]

Is there an easy way to add tests for these signature changes?

[weirdan]

You also need to update either the delta files or the historic callmap. See https://psalm.dev/docs/contributing/editing_callmaps/