TINYGL: glBlendFunc Implementation

This PR expands the functionality of TinyGL by adding support to glBlendFunc.
This function enables the user to implement effects such as alpha blending.
The grim engine was also updated to support the feature (and also to support sprite colorization for partial dimming of sprites)
I can confirm that `writePixel` and other FORCEINLINE functions are always inlined for
```
Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn)
Target: x86_64-apple-darwin12.5.0
Thread model: posix
```
(verified with `otool -tV residualvm | grep 'call.*writePixel'`)