Fix WebXR on desktop emulator by resetting active texture unit

[dsnopek]

This is such a dumb change, that I'm astonished that it fixes anything :-)

By selectively commenting out code, I found that it was RasterizerCanvasGLES3::reset_canvas() that ran when drawing 2D which fixed WebXR. So, I copied everything from that function into the end of RasterizerSceneGLES3::render_scene(), and by selectively commenting out those lines, I was able to narrow it down to this one line.

It seems super weird that changing the active texture unit fixes anything! It must mean that there's code somewhere else that's doing something with textures without remembering to set the active texture unit first? I looked around a little bit but couldn't identify what it could be.

Fixes #84255

[clayjohn]

This is generally a safe change to make. So let's go ahead for 4.2.

It would be ideal to track down where exactly we are modifying a texture without setting the active texture index first. But in the interest of fixing the regression, let's merge this ASAP

[akien-mga]

Thanks!