Capture ImGui in screenshots (scp-fs2open#7236)

* capture imgui in screenshots

* revert buffer changes

* request a screenshot for end of frame

* full revert for cleanliness

Author: MjnMixael

code/graphics/2d.cpp

@@ -98,6 +98,7 @@ color_gun Gr_t_red, Gr_t_green, Gr_t_blue, Gr_t_alpha;
 color_gun Gr_ta_red, Gr_ta_green, Gr_ta_blue, Gr_ta_alpha;
 color_gun *Gr_current_red, *Gr_current_green, *Gr_current_blue, *Gr_current_alpha;
 
+static SCP_string Pending_screenshot_filename;
 
 ubyte Gr_original_palette[768];		// The palette 
 ubyte Gr_current_palette[768];
@@ -2943,13 +2944,25 @@ void gr_flip(bool execute_scripting)
 
 	TRACE_SCOPE(tracing::PageFlip);
 
+	if (!Pending_screenshot_filename.empty()) {
+		gr_print_screen(Pending_screenshot_filename.c_str());
+		Pending_screenshot_filename.clear();
+	}
+
 	//Prevent a real page flip if OpenXR is on and claims that that wasn't the full image yet
 	if (!gr_openxr_flip()) {
 		gr_screen.gf_flip();
 		gr_setup_frame();
 	}
 }
 
+void gr_request_screenshot(const char* filename)
+{
+	if (Pending_screenshot_filename.empty()) {
+		Pending_screenshot_filename = filename;
+	}
+}
+
 void gr_print_timestamp(int x, int y, fix timestamp, int resize_mode)
 {
 	int seconds = f2i(timestamp);

code/graphics/2d.h

@@ -1074,6 +1074,8 @@ extern void gr_activate(int active);
 #define gr_blob_screen		GR_CALL(gr_screen.gf_blob_screen)
 #define gr_dump_envmap		GR_CALL(gr_screen.gf_dump_envmap)
 
+void gr_request_screenshot(const char* filename);
+
 //#define gr_flip				GR_CALL(gr_screen.gf_flip)
 void gr_flip(bool execute_scripting = true);
 

freespace2/freespace.cpp

@@ -4777,7 +4777,7 @@ int game_poll()
 				}
 
 				mprintf(( "Dumping screen to '%s'\n", tmp_name ));
-				gr_print_screen(tmp_name);
+				gr_request_screenshot(tmp_name);
 
 				os_config_write_uint(nullptr, "ScreenshotNum", counter);
 			}