refactor SDL out of core files

src/pxl8_hal.h

@@ -0,0 +1,18 @@
+#pragma once
+
+#include "pxl8_types.h"
+
+typedef struct pxl8_hal {
+    void* (*create)(pxl8_color_mode mode, pxl8_resolution res,
+                    const char* title, i32 win_w, i32 win_h);
+    void (*destroy)(void* platform_data);
+
+    u64 (*get_ticks)(void);
+
+    void (*present)(void* platform_data);
+    void (*upload_framebuffer)(void* platform_data, const u8* fb,
+                               i32 w, i32 h, const u32* palette,
+                               pxl8_color_mode mode);
+} pxl8_hal;
+
+extern const pxl8_hal pxl8_hal_sdl3;