Revert "Small improvement - removed field MultiImageSource._nextTexture, use local variables instead". This solves asynchronous loading issue (better than previous version)

This reverts commit 07997fb1cbbb3e3c4fa4d48cbb9861dbd0142c03.

Conflicts:
	MediaPortal/Source/UI/SkinEngine/Controls/ImageSources/MultiImageSource.cs
Fixed NRE in TextureImageSource