Continuous scales with manual breaks have incorrect legend ticks

[anthonynorth]

Legend ticks for continuous colour scales aren't functioning correctly when breaks are supplied. This is noticeable when breaks produce a piecewise function which greatly alters the colour mapping.

These should not produce the same legend:

# auto-breaks
scale_color_power(col, limits = -3:3)
# manual-linear-breaks
scale_color_power(col, limits = -3:3, breaks = 0:3)
# manual-breaks-power
scale_color_power(col, limits = -3:3, breaks = breaks_power()(0:3))

These scales have the following form when applied to a 0:1 numeric range:

Expected colour legend something like: