fix ofImage channel swap warning 

Every time a grayscale texture is loaded from gray pixels, the current implementation attempts to swap rgb values. This does not make sense on a grayscale image with only 1 channel. And correctly, the swap rgb method for grayscale pixels is a no-op.

Nevertheless, it leads to a warning on every load: `rgb swap not supported for this pixel format`. 

The warning is nice, but obvious to the point of being misleading.

This PR removes the warning message from the no-op swap pixels case.