Update the Add Grain effect to use GPU and 32-bit

I need to apply grain frequently in my work to clean plates I’ve created, and it needs to match the 6k/4k HDR plates I’ve been provided. I prefer Add Grain to Match Grain, as it’s quicker and more accurate for me to manually set it. Because of the size of the plates, and the lack of GPU acceleration in this plug-in, it’s the main bottleneck in my rendering speed, plus 32-bit would help with color fidelity. Obviously, multiframe rendering would be great, too, but I would at least love GPU acceleration to start!