A whole bunch of image-gen optimizations/fixes/cleanup

• Added wrapping/clamping option to `genDestImagePixelBytesAtPosition()` as `GEN_DEST_IMAGE_WRAP_ST` define.
	◦ When wrapping, takes the texel XY values and runs them through a modulo with the width/height.
	◦ When clamping, just clamps the texel XY values to 0..width/height.
• New `kBytesPerPixel` constant used in place of `4` all over.
• Fixed color stripes— I had forgotten to multiply the result of `nearestTexelXY[1] * self->_srcWidth + nearestTexelXY[0]` by 4 (`kBytesPerPixel`); now the calculation goes into a `texelIndex` var and the `* kBytesPerPixel` happens within the `CFDataGetBytes()` call.
• Minor: Using plain `int`s for widths/heights, instead of `size_t` to avoid unnecessary casts down the line.
• Minor: Prefixed every local in `genDestImagePixelBytesAtPosition()` that is only set-once-then-used with `const`— this helps the compiler know what to have ready when or something justifying like that I don’t really know.
• `surfaceSTToTexelUV()`’s sum calculation adjusted to first subtract the minimum dist (closest point) from each dist.
	» Just a test.  Not sure if this will help the mostly-greater-than-1.0 ST value issue, but it’s a shot.
• Minor: Wrapped all of the helper functions at the top of `ViewController.m` with `#pragma clang diagnostic ignored "-Wunneeded-internal-declaration”` so that they’re there only when we want them.