Fix screen redraws during effects

Normal redraws can overwrite effect frames with gfx_texture, causing
brief flashes during fade-out. Suppress redraws while an effect is
running and clear pending redraws at effect boundaries.

src/effect.c

@@ -191,6 +191,7 @@ static void eff_init(struct effect *eff, SDL_Rect *rect, EffectTexture *old, Eff
 
 	if (!eff->is_fullscreen)
 		gfx_updateScreen();  // Flush pending display changes.
+	gfx_setEffectRunning(true);
 }
 
 static void eff_finish(struct effect *eff, bool present) {
@@ -1912,4 +1913,5 @@ void effect_step(struct effect *eff, float progress) {
 
 void effect_finish(struct effect *eff) {
 	eff->finish(eff);
+	gfx_setEffectRunning(false);
 }

src/gfx_draw.c

@@ -45,6 +45,7 @@
 #include "debugger.h"
 
 static bool gfx_dirty;
+static bool gfx_effect_running;
 
 static void gfx_pal_check(void) {
 	if (nact->ags.pal_changed) {
@@ -64,7 +65,7 @@ SDL_Color gfx_getPaletteColor(uint8_t color) {
 }
 
 void gfx_updateScreen(void) {
-	if (!gfx_dirty)
+	if (!gfx_dirty || gfx_effect_running)
 		return;
 	SDL_RenderClear(gfx_renderer);
 	SDL_RenderCopy(gfx_renderer, gfx_texture, NULL, NULL);
@@ -73,6 +74,12 @@ void gfx_updateScreen(void) {
 	gfx_dirty = false;
 }
 
+void gfx_setEffectRunning(bool running) {
+	gfx_effect_running = running;
+	// Effects present directly. Do not replay pending redraws over their frames.
+	gfx_dirty = false;
+}
+
 void gfx_requestRedraw(void) {
 	gfx_dirty = true;
 }

src/gfx_private.h

@@ -37,6 +37,8 @@ extern surface_t *gfx_dibinfo;
 extern int view_w;
 extern int view_h;
 
+void gfx_setEffectRunning(bool running);
+
 SDL_Surface *gfx_dib_to_surface_with_alpha(int x, int y, int w, int h);
 SDL_Surface *gfx_dib_to_surface_colorkey(int x, int y, int w, int h, int col);