guiScalingFilter: Fix most memory leaks (minetest#12256)

Calls to the cache function ended up creating a new texture regardless whether
the texture is already cached.

src/client/client.cpp

@@ -334,6 +334,8 @@ Client::~Client()
  // cleanup 3d model meshes on client shutdown
  m_rendering_engine->cleanupMeshCache();
 
+ guiScalingCacheClear();
+
  delete m_minimap;
  m_minimap = nullptr;
 

src/client/guiscalingfilter.cpp

@@ -43,6 +43,10 @@ void guiScalingCache(const io::path &key, video::IVideoDriver *driver, video::II
 {
  if (!g_settings->getBool("gui_scaling_filter"))
  return;
+
+ if (g_imgCache.find(key) != g_imgCache.end())
+ return; // Already cached.
+
  video::IImage *copied = driver->createImage(value->getColorFormat(),
  value->getDimension());
  value->copyTo(copied);
@@ -90,14 +94,16 @@ video::ITexture *guiScalingResizeCached(video::IVideoDriver *driver,
  io::path scalename = origname + "@guiScalingFilter:" + rectstr;
 
  // Search for existing scaled texture.
- video::ITexture *scaled = g_txrCache[scalename];
+ auto it_txr = g_txrCache.find(scalename);
+ video::ITexture *scaled = (it_txr != g_txrCache.end()) ? it_txr->second : nullptr;
  if (scaled)
  return scaled;
 
  // Try to find the texture converted to an image in the cache.
  // If the image was not found, try to extract it from the texture.
- video::IImage* srcimg = g_imgCache[origname];
- if (srcimg == NULL) {
+ auto it_img = g_imgCache.find(origname);
+ video::IImage *srcimg = (it_img != g_imgCache.end()) ? it_img->second : nullptr;
+ if (!srcimg) {
  if (!g_settings->getBool("gui_scaling_filter_txr2img"))
  return src;
  srcimg = driver->createImageFromData(src->getColorFormat(),