pix_set: work with floating point images

[umlaeute]

there is some very basic support (a few get/set functions) for floating point images in Gem:

• [pix_texture], [pix_snap]
• [pix_sig2pix~], [pix_pix2sig~]
• [pix_dump]

it would be good if [pix_set] also supported floating point pixels