Fix painting on none 256 aligned images.

Internally the update tiles are 256x256. Due to some miscalculations
tiles were not generated correctly if the dimension of the image wasn't
a multifold of 256.

source/blender/blenkernel/intern/image_partial_update.cc

@@ -213,8 +213,8 @@ struct TileChangeset {
     tile_width = image_buffer->x;
     tile_height = image_buffer->y;
 
-    int chunk_x_len = tile_width / CHUNK_SIZE;
-    int chunk_y_len = tile_height / CHUNK_SIZE;
+    int chunk_x_len = (tile_width + CHUNK_SIZE - 1) / CHUNK_SIZE;
+    int chunk_y_len = (tile_height + CHUNK_SIZE - 1) / CHUNK_SIZE;
     init_chunks(chunk_x_len, chunk_y_len);
     return true;
   }

source/blender/draw/engines/image/image_drawing_mode.hh

@@ -229,7 +229,21 @@ template<typename TextureMethod> class ScreenSpaceDrawingMode : public AbstractD
     BLI_assert(float_buffer->rect == nullptr);
     BLI_assert(src->rect_float == nullptr);
     BLI_assert(src->rect != nullptr);
-    IMB_float_from_rect_ex(float_buffer, src, &iterator.changed_region.region);
+
+    /* Calculate the overlap between the updated region and the buffer size. Partial Update Checker
+     * always returns a tile (256x256). Which could lay partially outside the buffer when using
+     * different resolutions.
+     */
+    rcti buffer_rect;
+    BLI_rcti_init(&buffer_rect, 0, float_buffer->x, 0, float_buffer->y);
+    rcti clipped_update_region;
+    const bool has_overlap = BLI_rcti_isect(
+        &buffer_rect, &iterator.changed_region.region, &clipped_update_region);
+    if (!has_overlap) {
+      return;
+    }
+
+    IMB_float_from_rect_ex(float_buffer, src, &clipped_update_region);
   }
 
   void do_partial_update(PartialUpdateChecker<ImageTileData>::CollectResult &iterator,