Fix to gamma adjust pixels of user screenshots when not using shaders.

@@ -2476,7 +2476,7 @@ static int sdl_redraw_screen_for_screenshot(lua_State *L)
 	if (for_savefile)
 		redraw_now(redraw_type_savefile_screenshot);
 	else
-		redraw_now(redraw_type_screenshot);
+		redraw_now(redraw_type_user_screenshot);
 	return 0;
 }
 
@@ -2987,11 +2987,33 @@ static int sdl_set_gamma(lua_State *L)
 	return 1;
 }
 
+static void screenshot_apply_gamma(png_byte *image, unsigned long width, unsigned long height)
+{
+	// User screenshots (but not saved game screenshots) should have gamma applied.
+	if (gamma_correction != 1.0 && get_current_redraw_type() == redraw_type_user_screenshot)
+	{
+		Uint16 ramp16[256];
+		png_byte ramp8[256];
+		unsigned long i;
+
+		// This is sufficient for the simple gamma adjustment used above.
+		// If that changes, we may need to query the gamma ramp.
+		SDL_CalculateGammaRamp(gamma_correction, ramp16);
+		for (i = 0; i < 256; i++)
+			ramp8[i] = ramp16[i] / 256;
+
+		// Red, green and blue component are all the same for simple gamma.
+		for (i = 0; i < width * height * 3; i++)
+			image[i] = ramp8[image[i]];
+	}
+}
+
 static void png_write_data_fn(png_structp png_ptr, png_bytep data, png_size_t length)
 {
 	luaL_Buffer *B = (luaL_Buffer*)png_get_io_ptr(png_ptr);
 	luaL_addlstring(B, data, length);
 }
+
 static void png_output_flush_fn(png_structp png_ptr)
 {
 }
@@ -3065,6 +3087,7 @@ static int sdl_get_png_screenshot(lua_State *L)
 
 	glPixelStorei(GL_PACK_ALIGNMENT, 1);
 	glReadPixels(x, y, width, height, GL_RGB, GL_UNSIGNED_BYTE, (GLvoid *)image);
+	screenshot_apply_gamma(image, width, height);
 
 	for (i = 0; i < height; i++)
 	{

@@ -746,7 +746,7 @@ void on_redraw()
 
 	switch (current_redraw_type)
 	{
-	case redraw_type_screenshot:
+	case redraw_type_user_screenshot:
 	case redraw_type_savefile_screenshot:
 		// glReadPixels reads from the back buffer, so skip swap when we're doing a screenshot.
 		break;
@@ -789,7 +789,6 @@ void redraw_now(redraw_type_t rtype)
 
 	current_redraw_type = rtype;
 	on_redraw();
-	current_redraw_type = redraw_type_normal;
 
 	if (changed_gamma)
 	{
@@ -1652,6 +1651,7 @@ int main(int argc, char *argv[])
 				{
 				case 0:
 					if (isActive) {
+						current_redraw_type = redraw_type_normal;
 						on_redraw();
 						SDL_mutexP(renderingLock);
 						redraw_pending = 0;

@@ -56,7 +56,7 @@ extern void do_resize(int w, int h, bool fullscreen, bool borderless, float zoom
 
 typedef enum {
 	redraw_type_normal,
-	redraw_type_screenshot,
+	redraw_type_user_screenshot,
 	redraw_type_savefile_screenshot
 } redraw_type_t;