MP1-4307 : Synchronize access to textureData (attempt to fix AccessViolation errors)
MP1-4307 : Cleaned up all locks & code on C# side
Merge remote-tracking branch 'remotes/upstream/MP1-4307-BUG-fontEngine_crash_trigger_by_thirdparty_plugins_using_direct_call_in_GUITextureManager' into 1.6.0Fork-BDJ

Conflicts:
	mediaportal/Core/guilib/CachedTexture.cs
	mediaportal/Core/guilib/GUIImage.cs