GPv3 build modifier in "Nature Drawing Speed" mode didn't finish
building a frame when the time it took to draw those strokes by hand is
greater than the frame duration. Previous fix#129894 is only effective
for "Number of Frames" build mode. This fix moved the timing scaling
into `get_build_factor` and `get_factor_from_draw_speed` for more
granulated control in different modes.
Pull Request: https://projects.blender.org/blender/blender/pulls/130199
911ccac5f8