Fix grayscale images being too dark

[emilk]

What

• Closes Grayscale image presented too dark #3994

We previously would apply a grayscale colormap to a grayscale image - this was not only a bit redundant; it actually caused the wrong colors. Now we only apply the grayscale color map for things like depth images and tensors (when the user has selected such a colormap) and in all other cases we preserve the raw grayscale values as best we can.

We tested:

• grayscale images of different bit depth and ranges (see below)
• tensor selection colormap preview
• tensor with grayscale colormap
• depth point cloud selection colormap preview
• depth point cloud with grayscale colormap
• NV12 test

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)
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[Wumpf]

👍
once and for all!