Make TempImage act a bit more robustly.

Common/Render/ManagedTexture.cpp

@@ -54,11 +54,6 @@ private:
 	ManagedTexture::LoadState *state_;
 };
 
-TempImage::~TempImage() {
-	// Make sure you haven't forgotten to call Free.
-	_dbg_assert_(levels[0] == nullptr);
-}
-
 static Draw::DataFormat ZimToT3DFormat(int zim) {
 	switch (zim) {
 	case ZIM_RGBA8888: return Draw::DataFormat::R8G8B8A8_UNORM;

Common/Render/ManagedTexture.h

@@ -23,7 +23,13 @@ class LimitedWaitable;
 // The reason for the separation is so that the image can be loaded and decompressed on a thread,
 // and then only uploaded to the GPU on the main thread.
 struct TempImage {
-	~TempImage();
+	TempImage() = default;
+	// Owns levels[0], so no copies.
+	TempImage(const TempImage &) = delete;
+	TempImage &operator=(const TempImage &) = delete;
+	~TempImage() {
+		Free();
+	}
 	Draw::DataFormat fmt = Draw::DataFormat::UNDEFINED;
 	ImageFileType type = ImageFileType::UNKNOWN;
 	uint8_t *levels[16]{};   // only free the first pointer, they all point to the same buffer.