Particles 3D, Shape mode "points" and "directed points": impossible to move emission point in negative axis

[kubecz3k]

Operating system or device - Godot version:
e93ce33

Issue description:
With R/G/B color pixels we can code position of emission points with those emission shape modes: points' and 'directed points. I Haven't found any way to move emission point behind the spatial r/g/b can only be used to move in positive xyz axis. Some way to put those points in negative axis would be very helpful to position various effects in a proper way. Not sure how #10235 will be resolved, but if we would have additional scaling factor then maybe positions range could be defined in color from -1 to 1 and multiples by position scaling factor.

Steps to reproduce:

Link to minimal example project:

[kubecz3k]

It turns out it's working as expected, the only problem is with the fact it's impossible to define negative color channel values in graphic software. More info and workaround in #10233