Fix Exposure Warning incoherent behaviour

When using the "Exposure Warning" Tool to set maxium Shadow or highlight Input (with levels or curves, the behaviour of different for highlight than shadows (when selected).

Highlights exposure warning become red (or the color choosed) as soon as one of the color channel reaches the maximum value configured in settings/exposure (250 by default).

Shadows in contrary, when enabled in settings/exposure, become blue (or the color selected) ad soon all the three color channel have reached the minmal value (zéro by default).

One or the other behavior are correct, but both together are confiusing and unlogical.

Personnaly, I do prefer when the Exposure Warning shows up when all the channel reach 0-0-0 (for general purpose) but I deeply would like to see a way to have color clipping channel by channel (as we have in Photoshop) for fine works tunning and as I suggested in my "Improve Capture One ideas": Improve Levels tool by visually showing clipping using alt key.