Fix #115043: Compositor crashes for huge render sizes

The experimental GPU compositor crashes when the render size is huge.
This is just due to GPU texture allocation failing. The patch fixes that
by downscaling the render result when reading, then upscaling it again
when writing. Additionally, the render size was adapted to the
downscaled size since it is used by other input nodes. This is not an
ideal solution, but it a good temporary solution to prevent crashes
until we have proper support for huge textures.

@@ -1206,0 +1222,4 @@
+  /* There is no way to know if the render size is too large except if we actually allocate a test
+   * texture, which we want to avoid due its cost. So we employ a heuristic that so far has worked
+   * with all known GPU drivers. */
+  if (width * height > (max_texture_size * max_texture_size) / 4) {

@@ -1206,0 +1225,4 @@
+  if (width * height > (max_texture_size * max_texture_size) / 4) {
+    BKE_report(render->reports,
+               RPT_ERROR_OUT_OF_MEMORY,
+               "Render size too large for GPU! Use CPU compositor instead.");