Texture loading/exporting bug when overriding driver_name for mobile

[ProbDenis]

Godot version:
3.1 RC1

Issue description:
I have a game that is supposed to use GLES3 by default, but GLES2 for mobile. So I override the renderer_name option in the project settings, for mobile, while the default remains GLES3.
When I do that, the exported build crashes immediately - the debug output shows that it fails to load ETC textures. If I switch the default renderer to GLES2, I don't get these error messages and the game launches (but it crashes later for a different reason, see #26902)

So I assume that if you override the renderer for mobile and set it to GLES2, the game is still exported as if it was a GLES3 game, so the engine doesn't find the correct texture files. But if the GLES2 is the project default, the textures are exported correctly. I guess that setting fallback_to_gles2 would fix it, but there's no point in exporting both ETC and ETC2 textures if I want to force GLES2 on mobile devices.

Minimal reproduction project:
Any demo that uses VRAM compression for some textures. Use GLES3 as default and override to GLES2 for mobile.

[akien-mga]

Did you enable import of ETC textures? For the use case you describe, if you use GLES3 on desktop and GLES2 on mobile, you have to enable import_etc, disable import_etc2 (doesn't hurt to keep it, but you don't need it) and enable import_s3tc (for desktop). It should work fine when exporting.

[Calinou]

@ProbDenis Can you reproduce this with Godot 3.2.1? See akien-mga's comment above.