Added GLBuffer.Float.add(int).

GL20Buffer (used by the Java, GWT and Android backends) adds the float directly
to the byte buffer via an IntBuffer pointing to the same byte array as the
FloatBuffer. The GWT backend benefits greatly from this because otherwise it
has to use the monstrously slow manual implementation of Float.intBitsToFloat.

IOSGLBuffer still uses Float.intBitsToFloat (which is an intrinsic in IKVM and
turns into very fast/optimizable assembly code).
Use the new GLBuffer.Float.add(int).