Equal axes

[brenocq]

Right now the zoom of each axis is independent, but we should be able to force the axes to zoom together. This task would introduce the ImPlot3D_Equal flag to the plot flags.

[dimateos]

Probably what may confuse most users is the linked zoom for 2 simultaneous dimensions when the mouse is hovering a plane.
I like being able to zoom per axis, but maybe I prefer to zoom over all when hovering no axis. So I guess I would add also the flag ImPlot3D_Disable2DZoom (with a better name picked by u xD)

Btw there is a lot of potential for modifier keys as suggested (kind of) here #38
So for example the zoom could be global by default, and per axis/plane only when the user is holding CTRL or SHIFT