Fix occasional long ticks

[ffreyer]

I didn't look into the details, but I think optimize_ticks used to convert any input to Float64 when calculating ticks. This was changed in JuliaPlots/PlotUtils.jl#124 so a Float32 input now results in Float32 ticks. Occasionally ticks are off by a bit, e.g.

ticks = PlotUtils.optimize_ticks(0.25314125f0, 0.94421935f0, k_min = 4, k_max = 8)[1]
# Float32[0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.90000004]

This also happened before, but on the Float64 level.

Showoff.jl ignored the single bit differences in Float64, but not in Float32, so now we get long tick labels:

Showoff.showoff(Float32[0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.90000004], :plain)
# ["0.30000001", "0.40000001", "0.50000000", "0.60000002", "0.69999999", "0.80000001", "0.90000004"]

See JuliaGraphics/Showoff.jl#44

As a quick fix converting our ticks to Float64 before passing them should work.

Example from CI in #1512: (This is short_test_67.png)