Document the histogram/range in the framebuffer

This is particularly for the Depth Buffer in the Framebuffer pane. Sometimes I can't see certain objects rendering into the depth buffer because it is being visualized from 0 to 1. If most of the buffer is closer to 0 than 1, I can't really see anything. So I'd like to be able to clamp the visual range in order to _more_ easily visualize the framebuffer.

I guess it's a bit tricky for how the color channels are surfaced, because the depth buffer isn't really visualized as a single channel.