[rcore][SDL] Accumulate relative mouse motion per frame

[snowyukitty]

With the cursor disabled, the SDL backend keeps only the last mouse-motion event polled in a frame. This loses movement when several events arrive between frames, making camera rotation depend on the frame rate.

Accumulate each event's relative displacement. Reset both mouse positions before polling a relative frame, so an empty first frame after DisableCursor() also returns zero instead of a stale absolute delta. Absolute cursor movement continues to use the latest position.

Fixes #6168.

Validation on Linux/Xvfb with real SDL event queues and the public GetMouseDelta() API:

• SDL2: all 11 scenarios pass; the upstream baseline fails four. SDL3 3.4.18: all 12 pass; the baseline fails five, including fractional motion.
• The same 64 input events produce (128, -64) both in one frame and across 64 frames. The baseline returns (2, -1) when they are batched.
• Covered multiple/opposing/single/zero events, empty frames, switching into relative mode and back to absolute mode.
• Windows x64 SDL2 2.32.10: MinGW cross-compilation and executable linking passed. Native Windows runtime testing was not performed.
• The existing Clang-Tidy workflow reports the same three warnings in untouched rcore_drm.c on the baseline and patch. Existing SDL unused-function compiler warnings remain.

Minimal SDL2 event-queue reproduction

Build raylib with PLATFORM=PLATFORM_DESKTOP_SDL, then link this with raylib and SDL2. Run in an X11 session (or Xvfb). The expected delta is (128, -64).

#include "raylib.h"
#include <SDL.h>
#include <stdio.h>

extern void PollInputEvents(void);

int main(void)
{
    InitWindow(320, 240, "Relative mouse motion");
    DisableCursor();
    SDL_PumpEvents();
    SDL_FlushEvents(0, SDL_LASTEVENT);
    PollInputEvents();
    for (int i = 0; i < 64; i++)
    {
        SDL_Event event = { 0 };
        event.type = SDL_MOUSEMOTION;
        event.motion.windowID = 1;
        event.motion.xrel = 2;
        event.motion.yrel = -1;
        SDL_PushEvent(&event);
    }
    PollInputEvents();
    Vector2 delta = GetMouseDelta();
    printf("delta=(%.0f, %.0f)\n", delta.x, delta.y);
    CloseWindow();
    return 0;
}

[raysan5]

@snowyukitty thanks for the review

[goob228]

Cool that you also fixed another issue that I thought about by zeroing CORE.Input.Mouse.previousPosition before while loop. Thanks