Separate particles material from 2D and 3D due to properties bounds inconsistencies

[Ranoller]

Godot version:

Godot 3 upt to master

OS/device including version:

All

Issue description:

From godot 3 and the addition of particles material, 2D and 3D share same code. Properties bounds that works well or have a significative impact in 3D doesn´t have much effect in 2D and backwards (bounds seems to be programmed with 3D in mind). I was looking for change this bounds in:

scene\3d\particles.cpp:
void ParticlesMaterial::_bind_methods()

But if i change that for do feasible some effects that with actual bounds you can´t get in 2D... well, parameter-bound for 3D become oversized. This can be accomplished by separating both codes... I do not feel capable to do that, so this is a ... feature request?

But if nobody have problem with upper bounds for 3D (Doesn´t break compatibility, only can make crazy effects in the upper side of the bound....), I can change directly this code.

[Calinou]

Closing in favor of godotengine/godot-proposals#815, as feature proposals are now tracked on the Godot proposals repository.