Custom axis ratios

[brenocq]

ImPlot3D currently uses a default cube-like aspect ratio for its 3D plots, where all axes have equal scaling. However, not all data fits neatly into a cube! With custom axis ratios we would be able to define the aspect ratio for each axis (e.g., 1:2:3 or 1:1:2).

Some examples:

[brenocq]

@epezent feedback in #24

Consider allowing the user to define the shape of the 3D plot (i.e. it is currently a cube; what if I want a prism or to enlongate one axis?)

Moving priority to high!

[brenocq]

This feature was merged in #46