Add an original size constant.

I'm still defaulting to screen dimensions when given wrap_content for Views on the theory that, on average, downsampling to around the screen size and providing an image that can be displayed (isn't over the max gl texture size) and that won't blow through huge quantities of memory is preferable, despite the occasional issue with cropping (see #151). Although this is safer from a display/memory perspective, it's also less correct. You could definitely make a reasonable argument that ``WRAP_CONTENT`` should be treated as ``SIZE_ORIGINAL``.

All that said, I did included fixes for #135, so if ``WRAP_CONTENT`` is set for only one of width/height, we will only substitute in the screen size for the single dimension. I'd expect that change will at least mitigate some of the issues people have raised in #151 and #263.
The "Trying to load image into ... using WRAP_CONTENT" warning is still valid if both sides are wrap, isn't it?