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saveImage and saveImageMem read the channel buffers back as float regardless of the input type, so a u8 image (or f64, or any non-f32 type) was written as garbage, all zeros in the reported case. The input is now cast to f32 after the range scaling, and the docs say which types and value range are accepted. Fixes #3544.
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saveImage and saveImageMem read their channel buffers back as float whatever the input type, so a u8 image (or f64, or any non-f32 type) was written as garbage, all zeros in the reported case (#3544). The input is now cast to f32 after the existing range scaling, and the saveImage docs say any real type is accepted with values in [0, 255] (a maximum of at most 1 is scaled by 255, as before). Adds a round-trip test through a PNG file and the in-memory path for u8, u16, s16, s32, u32 and f64.
Verified on the CPU backend and on OpenCL (Intel Arc B580 and UHD 770): the new test fails on master and passes with the fix, and the imageio suite passes on all three.