ImGui_ImplAllegro5_UpdateMouseCursor should not create a new cursor each frame

[Helodity]

Currently, ImGui sends a call to Allegro each time a frame is drawn, either reusing a stored invisible cursor or creating a new system cursor.

Allegro currently has a memory leak in its X11 interface (see this Allegro PR) where system cursors are not cleared when a new one is generated. This paired with creating a new cursor each frame results in a leak that results in an XWayland abort within 10 minutes (see this xorg bug report)

This fix reduces the amount of unnecessary cursor creations by caching which cursor is currently being rendered, and only creating a new one when it changes. This does not fix the memory leak, but heavily mitigates its effect.

[ocornut]

Thank you for the PR.
Indeed it doesn't seem like there is a way to call the equivalent of al_create_mouse_cursor for a system cursor so I cannot think of another workaround, it would be work reporting to Allegro repository.

I reworked and pushed your fix as 2d2c7d3 (fixed the coding-style to match what's in the file).

Thanks!