trigger VAE tiling by image area instead of dimensions

I've tested with GGML_VULKAN_MEMORY_DEBUG all resolutions with
the same 768x768 area (even extremes like 64x9216), and many
below that: all consistently allocate 6656 bytes per image pixel.
As tiling is primarily useful to avoid excessive memory usage, it
seems reasonable to enable VAE tiling based on area rather than
maximum image side.

However, as there is currently no user interface option to change
it back to a lower value, it's best to maintain the default
behavior for now.

Author: wbruna

otherarch/sdcpp/sdtype_adapter.cpp

@@ -483,17 +483,24 @@ sd_generation_outputs sdtype_generate(const sd_generation_inputs inputs)
     }
 
     int tiled_vae_threshold;
+    bool dotile;
     if (cfg_tiled_vae_threshold == 0) {
+        // Legacy behavior: limit by image dimensions
         tiled_vae_threshold = 768;
+        dotile = (sd_params->width>tiled_vae_threshold || sd_params->height>tiled_vae_threshold);
     } else {
         if (cfg_tiled_vae_threshold > 0) {
             tiled_vae_threshold = cfg_tiled_vae_threshold;
         } else {
             tiled_vae_threshold = 8192; // effectively avoids tiling
         }
+        // when explicitely set, limit by image area
+        // on the Vulkan backend, typically limited at 4G for a single allocation,
+        // the memory used for the VAE buffer is 6656 bytes per image pixel; a
+        // 768x768 square image (and all resolutions with the same area) stays a
+        // bit below that limit, at 3.66G.
+        dotile = (sd_params->width*sd_params->height > tiled_vae_threshold*tiled_vae_threshold);
     }
-
-    bool dotile = (sd_params->width>tiled_vae_threshold || sd_params->height>tiled_vae_threshold);
     set_sd_vae_tiling(sd_ctx,dotile); //changes vae tiling, prevents memory related crash/oom
 
     if (sd_params->clip_skip <= 0) {