Red channel weirdness when SUPPORT_HDR is disabled

[hrj]

This is something specific to the emscript port; it works fine on native.

Example : http://i.imgur.com/Oet69lD.png

Thoughts:

• The operand size in some write is probably bigger than expected, which is probably causing the red channel to be over-written. Try disabling / trapping some memory writes to track down the problem.
• Try with a new version of emscripten, when it comes out.

[hrj]

The latest version has HDR disabled and yet doesn't have this problem.