Colormap selection preview is sometimes bugged, sometimes not there at all

[teh-cmc]

Here's a colormap selector for depth clouds -- no preview at all:

And here's a colormap selector for tensors -- layout is broken:

[emilk]

So there are two problems here:

A) we have two different code paths for selecting a colormap, and only one has the preview
B) the preview is broken

• I wonder if this is related to Image zoom rectangle is off by one pixel #3074

[emilk]

The viewport given to new_renderer_callback for each gradient image is correct:

So the problem is either in

• how egui_wgpu handles the callback (maybe broke in Improved wgpu callbacks emilk/egui#3253)
• our rectangle shader (re_renderer::renderer::TexturedRect)
• our transforms (re_renderer::view_builder::TargetConfiguration)