Refactored stream adapters; fix for MOAIImageTexture invalidation

The stream refactor bears some attention. I wrote a sample to test it, but there are probably more test cases. Will eventually adapt that to run as an automated test.

The image texture invalidation is an alternative to InvalidateContents. The intent is to work through the usual gfx resource lifecycle but just to suppress the memory reporting when the texture gets recreated. (Though maybe it *shouldn't* get suppressed.)