Use separate texture unit for light_texture

[clayjohn]

Fixes: #42521

This bug has been present since the early stages of the GLES2 renderer. In a366d45 when reduz added the screen_texture he made it share a texture unit with the light texture. I am not sure why. As a result, when lights are drawn over sprites that read from SCREEN_TEXTURE, during the fragment pass they read from the light texture instead of the screen texture which results in a weird ghosting artifact.

About 30% of GLES2 devices have only 8 texture units so we try to use as few internal texture units as possible. That may be why reduz chose to share light texture and screen texture. This change makes the Godot internal shader take up 6 texture units which leaves the user with only 2 for themselves one these devices.

In light of the above, reduz needs to approve this change himself. It may have been an oversight, but there also may be a legitimate reason to avoid using the 6th texture unit.

[akien-mga]

In light of the above, reduz needs to approve this change himself. It may have been an oversight, but there also may be a legitimate reason to avoid using the 6th texture unit.

It seems it was an oversight and reduz agrees with the change, though he shares the same concern about the more limited GLES2 devices. But it's important to make engine features work properly IMO, and so far no user of such an old device complained about having only 3 custom texture slots available, so maybe 2 will be fine too (if anyone makes games for the lowest end GLES2 devices).

[akien-mga]

Thanks!