Fix mouse wheel not working in PLATFORM_RPI or PLATFORM_DRM

[ghost]

In the meantime (just to not leave the mouse wheel broken) I think the problem was sharing the same variable for holding both "the event" and "the current" value since they run independently.

GetMouseWheelMove() returns the CORE.Input.currentWheelMove (L3949-L3959). But that is always zeroed by PollInputEvents() (L5065) when it runs on EndDrawing(). So the event.value (L6680) never ends up staying on CORE.Input.currentWheelMove long enough before it gets zeroed. This could be solved by storing the event.value in a new CORE.Input.eventWheelMove (R471, R6682). And, after updating the CORE.Input.currentWheelMove (that is what GetMouseWheelMove() needs), reset the CORE.Input.eventWheelMove instead (L5065-R5067).

Tested the fix successfully for PLATFORM_DRM on Raspberry Pi 3 Model B 1.2 with Raspberry Pi OS (11 Bullseye) 32-bit armhf.

Fix #2660.

Edit: added line marks.

[raysan5]

@ubkp thank you very much for reviewing this issue! As always, a great description of the issue and a great fix! Your contributions to raylib are really appreciated!