Alpha blending not working correctly in GLES2

[kiidmale]

Godot version:

3.1 alpha5 b60939b

OS/device including version:

macOS Hight Sierra / Intel HD Graphics 5000

Issue description:

Blending multiple textures with alpha channel does not produce overlapping effects in GLES2.
You can see the differences as below.

Scene description
Left: CPUParticles2D with texture having alpha
Right: 10 godot icons with alpha drawn on the same position

GLES3

GLES2

Minimal reproduction project:

gles2-blend.zip

[kiidmale]

I think I found the cause of the issue. I'll submit PR soon.