BACKENDS: OPENGL: Fix getScreenFormat while running in 3D

[lephilousophe]

This is a small fix to allow engines to use the optimum textures when 3D rendering.

I also removed the format argument from loadVideoMode as we were never using it (in 2D and in 3D).

[ccawley2011]

It might be a good idea to move the pixel format to texture format mapping into common code to make this easier to make use of.

[tunnelsociety]

Build failure; need ifdef?

./configure ... --disable-opengl-game ...

C++      backends/graphics/opengl/opengl-graphics.o
backends/graphics/opengl/opengl-graphics.cpp:240:6: error: use of undeclared identifier '_renderer3d'
        if (_renderer3d) {
            ^
1 error generated.

[lephilousophe]

Indeed, fixing ASAP

[lephilousophe]

@tunnelsociety Should be fixed in master.

[tunnelsociety]

@lephilousophe Mercy buckets 😸