[rcore] Fix SUPPORT_WINMM_HIGHRES_TIMER for PLATFORM_DESKTOP_SDL

[ghost]

Context

• During the Christmas holidays a visiting relative was playing a game I've made with my kernel library on his very up-to-date notebook, but there were (low) framerates issues. Upon inspection the issue was the frame sleep time being too long on Windows. The problem was solved with SDL_HINT_TIMER_RESOLUTION which is kind of equivalent to Windows timeBeginPeriod()/timeEndPeriod().

• Since raylib's PLATFORM_DESKTOP_SDL doesn't use rcore.c's InitTimer() (L3040-L3047), it ends up having the same issue. And since this looks relevant enough, I'm sending this PR.

• This issue is more noticeable when vsync is turned off and a given frame cap is enforced.

Changes

1. Adds a SDL_HINT_TIMER_RESOLUTION hint to InitPlatform() (R1430-R1432) for PLATFORM_DESKTOP_SDL to handle the SUPPORT_WINMM_HIGHRES_TIMER option.

2. This is similar to how PLATFORM_DESKTOP handles it using Windows timeBeginPeriod()/timeEndPeriod() (L66-L70, L3040-L3047, L1590-L1596).

Edits

• 1: editing, added line marks.

[raysan5]

@ubkp Great improvement! Thank you very much!