sokol_app.h: initialize window with resizing disabled

[serkonda7]

For the @vlang project I need a config that keeps sokol windows at a fixed size (no resizing).
I tried calling sapp_consume_event() in the callback method until I found out it's not implemented on Desktop platforms.
My attempts to implement it in sokol failed, so it would be very nice if you could do it.

[floooh]

I think the best way to implement this is via a config flag in sapp_desc, and then when creating the window omit the platform-specific "resizable flag" (in case you want to hack this yourself for now).

Later it might make sense to add more such flags (e.g. for removing window chrome).