video-blend: fix blending 8-bit and 16-bit frames together

Replace hardcoded 255s with the correct max value for the given color depth. Use 64-bit integer in calculations where overflow may occur.

My test pipeline is

gst-launch-1.0 videotestsrc ! video/x-raw,format=I420_10LE ! textoverlay text="Testing testing 123" font-desc="Sans, 32" auto-resize=false ! videoconvert ! autovideosink

The image output with blended text overlay before and after the change: