Coordinates are already in terms of pixels -- no need to do content scaling
Remove SDL code from texture cache, and move all image loading code to CCImage.
Pre-multiply text when rendered by Emscripten's SDL implementation (since HTML5 canvas is non-pre-multiplied).
Add a flag to indicate whether raw image data is pre-multiplied or not, and switch to using Emscripten's SDL implementation when loading images from file (much faster)