Technically the regression was caused by #124472 which made it so
duplicating ImBuf allocates the exact amount of memory needed to
hold the pixels, while before IMB_dupImBuf() would leave the float
buffer over-allocated for images that are less than 4 channels per
pixel.
At the same time IMB_scalefastImBuf() was hard-coded to use 4
channels per pixels, for both byte and float buffers. It did not
crash in Blender 4.1 as it was accessing memory that is over-allocated,
but it also did not generate proper preview.
This fix makes the IMB_scalefastImBuf() to operate on an arbitrary
number of channels in the float buffer.
Pull Request: https://projects.blender.org/blender/blender/pulls/126234
1fc6a5b9bd