Fix GPUParticles2D wrong lifetime draw order on first particle

[ator-dev]

Fix #50825

[akien-mga]

Is that an issue in 3.x too, or only 4.0? And is it a problem for 3D particles?

[ator-dev]

@akien-mga In 3.x the Index sorting mode works the same way [correctly], Lifetime works correctly, and ReverseLifetime doesn't exist; I can't find the equivalent code though, it seems to work in quite a different way (seemingly, in 3.x draw_order is implemented in C++ instead of GLSL).
I've tried to test GPUParticles3D but can't work out how to test the draw order - I can only make the normal distance-based mesh rendering work without causing lots of errors. Would you mind telling me how to do it or testing it yourself please?

By the way, I just realised my commit is currently flawed. While it fixes the problem, when you open or run a scene already containing a particle emitter, there is a visual glitch (where new particles appear on top for a split second) that continues until you change its speed_scale property - even to the same value. I have no idea what's causing this and can't work out a fix as of yet.

[clayjohn]

Thank you for investigating this bug and attempting a fix. While evaluating your fix, I came up with a different solution that seems to avoid creating new artifacts.

Superseded by #55382