Fix T98462: Save Screenshot (glReadPixels) fails under Wayland

Use an off-screen buffer for the screen-shot operator.

Reading from the front-buffer immediately after calling swap-buffers
failed for GHOST/Wayland in some cases.
While EGL can request to preserve the front-buffer while drawing,
this isn't always supported. So workaround the problem by avoiding
use of the front-buffer entirely.

source/blender/editors/screen/screendump.c

@@ -54,13 +54,12 @@ static int screenshot_data_create(bContext *C, wmOperator *op, ScrArea *area)
 {
   int dumprect_size[2];
 
-  wmWindowManager *wm = CTX_wm_manager(C);
   wmWindow *win = CTX_wm_window(C);
 
   /* do redraw so we don't show popups/menus */
   WM_redraw_windows(C);
 
-  uint *dumprect = WM_window_pixels_read(wm, win, dumprect_size);
+  uint *dumprect = WM_window_pixels_read_offscreen(C, win, dumprect_size);
 
   if (dumprect) {
     ScreenshotData *scd = MEM_callocN(sizeof(ScreenshotData), "screenshot");

source/blender/windowmanager/WM_api.h

@@ -134,7 +134,24 @@ void WM_window_pixel_sample_read(const wmWindowManager *wm,
                                  const int pos[2],
                                  float r_col[3]);
 
+/**
+ * Read pixels from the front-buffer (fast).
+ *
+ * \note Internally this depends on the front-buffer state,
+ * for a slower but more reliable method of reading pixels, use #WM_window_pixels_read_offscreen.
+ * Fast pixel access may be preferred for file-save thumbnails.
+ *
+ * \warning Drawing (swap-buffers) immediately before calling this function causes
+ * the front-buffer state to be invalid under some EGL configurations.
+ */
 uint *WM_window_pixels_read(struct wmWindowManager *wm, struct wmWindow *win, int r_size[2]);
+/**
+ * Draw the window & read pixels from an off-screen buffer (slower than #WM_window_pixels_read).
+ *
+ * \note This is needed because the state of the front-buffer may be damaged
+ * (see in-line code comments for details).
+ */
+uint *WM_window_pixels_read_offscreen(struct bContext *C, struct wmWindow *win, int r_size[2]);
 
 /**
  * Support for native pixel size

source/blender/windowmanager/intern/wm_draw.c

@@ -1191,6 +1191,39 @@ static void wm_draw_surface(bContext *C, wmSurface *surface)
   wm_surface_clear_drawable();
 }
 
+uint *WM_window_pixels_read_offscreen(bContext *C, wmWindow *win, int r_size[2])
+{
+  /* NOTE(@campbellbarton): There is a problem reading the windows front-buffer after redrawing
+   * the window in some cases (typically to clear UI elements such as menus or search popup).
+   * With EGL `eglSurfaceAttrib(..)` may support setting the `EGL_SWAP_BEHAVIOR` attribute to
+   * `EGL_BUFFER_PRESERVED` however not all implementations support this.
+   * Requesting the ability with `EGL_SWAP_BEHAVIOR_PRESERVED_BIT` can even cause the EGL context
+   * not to initialize at all.
+   * Confusingly there are some cases where this *does* work, depending on the state of the window
+   * and prior calls to swap-buffers, however ensuring the state exactly as needed to satisfy a
+   * particular GPU back-end is fragile, see T98462.
+   *
+   * So provide an alternative to #WM_window_pixels_read that avoids using the front-buffer. */
+
+  /* Draw into an off-screen buffer and read it's contents. */
+  r_size[0] = WM_window_pixels_x(win);
+  r_size[1] = WM_window_pixels_y(win);
+
+  GPUOffScreen *offscreen = GPU_offscreen_create(r_size[0], r_size[1], false, GPU_RGBA8, NULL);
+  if (UNLIKELY(!offscreen)) {
+    return NULL;
+  }
+
+  const uint rect_len = r_size[0] * r_size[1];
+  uint *rect = MEM_mallocN(sizeof(*rect) * rect_len, __func__);
+  GPU_offscreen_bind(offscreen, false);
+  wm_draw_window_onscreen(C, win, -1);
+  GPU_offscreen_unbind(offscreen, false);
+  GPU_offscreen_read_pixels(offscreen, GPU_DATA_UBYTE, rect);
+  GPU_offscreen_free(offscreen);
+  return rect;
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */

source/blender/windowmanager/intern/wm_window.c

@@ -1951,6 +1951,9 @@ void WM_window_pixel_sample_read(const wmWindowManager *wm,
 
 uint *WM_window_pixels_read(wmWindowManager *wm, wmWindow *win, int r_size[2])
 {
+  /* WARNING: Reading from the front-buffer immediately after drawing may fail,
+   * for a slower but more reliable version of this function #WM_window_pixels_read_offscreen
+   * should be preferred. See it's comments for details on why it's needed, see also T98462. */
   bool setup_context = wm->windrawable != win;
 
   if (setup_context) {