vo_gl/sdl: use desktop resolution for fullscreen mode

Before the SDL code would change screen resolution when switching to
fullscreen. Try to keep existing desktop resolution instead.

libvo/gl_common.c

@@ -1961,6 +1961,7 @@ static int sdl_check_events(struct vo *vo)
     return res;
 }
 
+static void new_sdl_update_xinerama_info(struct vo *vo) { sdl_update_xinerama_info(); }
 static void new_vo_sdl_fullscreen(struct vo *vo) { vo_sdl_fullscreen(); }
 
 #endif
@@ -2048,6 +2049,7 @@ MPGLContext *init_mpglcontext(enum MPGLType type, struct vo *vo)
         SDL_Init(SDL_INIT_VIDEO);
         ctx->setGlWindow = setGlWindow_sdl;
         ctx->swapGlBuffers = swapGlBuffers_sdl;
+        ctx->update_xinerama_info = new_sdl_update_xinerama_info;
         ctx->check_events = sdl_check_events;
         ctx->fullscreen = new_vo_sdl_fullscreen;
         //the SDL code is hardcoded to use the deprecated vo API

libvo/sdl_common.c

@@ -25,11 +25,13 @@
 #include "input/keycodes.h"
 #include "input/input.h"
 #include "video_out.h"
+#include "aspect.h"
 
 static int old_w;
 static int old_h;
 static int mode_flags;
 static int reinit;
+static int screen_w, screen_h;
 
 int vo_sdl_init(void)
 {
@@ -39,6 +41,12 @@ int vo_sdl_init(void)
         SDL_Init(SDL_INIT_VIDEO|SDL_INIT_NOPARACHUTE) < 0)
         return 0;
 
+    if (screen_w == 0) {
+        const SDL_VideoInfo *vi = SDL_GetVideoInfo();
+        screen_w = vi->current_w;
+        screen_h = vi->current_h;
+    }
+
     // Setup Keyrepeats (500/30 are defaults)
     SDL_EnableKeyRepeat(SDL_DEFAULT_REPEAT_DELAY, 100 /*SDL_DEFAULT_REPEAT_INTERVAL*/);
 
@@ -59,6 +67,15 @@ void vo_sdl_uninit(void)
         SDL_QuitSubSystem(SDL_INIT_VIDEO);
 }
 
+void sdl_update_xinerama_info(void)
+{
+    if (vo_screenwidth <= 0 || vo_screenheight <= 0) {
+        vo_screenwidth  = screen_w;
+        vo_screenheight = screen_h;
+    }
+    aspect_save_screenres(vo_screenwidth, vo_screenheight);
+}
+
 void vo_sdl_fullscreen(void)
 {
     if (vo_fs) {

libvo/sdl_common.h

@@ -30,6 +30,7 @@
 
 int vo_sdl_init(void);
 void vo_sdl_uninit(void);
+void sdl_update_xinerama_info(void);
 void vo_sdl_fullscreen(void);
 int sdl_set_mode(int bpp, uint32_t flags);
 int sdl_default_handle_event(SDL_Event *event);