feat: add vsync-priority frame pacing to video backends

Present now dispatches through a function pointer chosen at init and
re-chosen by two new runtime setters: real vsync if requested and the
driver grants it, else a manual sleep-based cap (auto-detects display
refresh rate, falls back to 60fps if that can't be queried), else
fully uncapped. skele_video_using_vsync() reports what was actually
granted; skele_video_set_vsync()/skele_video_set_max_fps() let a game
change either live without tearing down the renderer/GL context.

include/client/video.h

@@ -12,10 +12,14 @@ extern "C" {
 #define SKELE_VIDEO_RESIZABLE 0x04
 #define SKELE_VIDEO_HIGHDPI 0x08
 #define SKELE_VIDEO_FULLSCREEN_EXCLUSIVE 0x10
+#define SKELE_VIDEO_VSYNC 0x20
 
 #define SKELE_DEFAULT_RENDER_WIDTH 320
 #define SKELE_DEFAULT_RENDER_HEIGHT 200
 
+#define SKELE_VIDEO_FPS_UNCAPPED UINT16_MAX
+#define SKELE_VIDEO_FPS_FALLBACK 60
+
 typedef enum {
 	SKELE_FULLSCREEN_BORDERLESS,
 	SKELE_FULLSCREEN_EXCLUSIVE
@@ -26,12 +30,16 @@ typedef struct {
 	uint16_t render_height;
 	uint16_t window_width;
 	uint16_t window_height;
+	uint16_t max_fps;
 	uint8_t flags;
 } skele_video_config_t;
 
 uint8_t skele_video_init(skele_video_config_t cfg);
 void skele_video_shutdown(void);
 void skele_video_present(void);
+uint8_t skele_video_using_vsync(void);
+void skele_video_set_vsync(uint8_t enabled);
+void skele_video_set_max_fps(uint16_t max_fps);
 void skele_video_set_title(const char *title);
 void skele_video_toggle_fullscreen(void);
 void skele_video_set_fullscreen_kind(skele_fullscreen_kind_t kind);