Model preview: AVA2 render-target window on a Vulkan colour target

Port of AVA2's CRenderTarget / GrpRenderTargetTexture / renderTarget module.
The host gains an RGBA8 offscreen colour pass for D3D colour render targets;
mt_preview shows wearable items on every allowed race and monsters with
motions, reached by X+click in the inventory and shop and from the wiki.
Player races are created as TYPE_PC so their parts exist.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/host/vk_textures.cpp

@@ -430,6 +430,19 @@ void VulkanWindow::prune_textures() {
             it = textures_.erase(it);
         } else ++it;
     }
+    // PORT: render targets too (a closed model preview); a later use recreates and clears one.
+    for (auto it = render_targets_.begin(); it != render_targets_.end();) {
+        if (frame_number_ - it->second.last_used_frame > static_retention_frames) {
+            for (auto pair = paired_descriptors_.begin(); pair != paired_descriptors_.end();) {
+                if (pair->first.find("\n" + it->first + "\n") != std::string::npos) {
+                    vkFreeDescriptorSets(device_, descriptor_pool_, 1, &pair->second.set);
+                    pair = paired_descriptors_.erase(pair);
+                } else ++pair;
+            }
+            release_render_target(it->second);
+            it = render_targets_.erase(it);
+        } else ++it;
+    }
 }
 
 } // namespace mt_host