Add bent normal map support for the compatibility renderer

[LunaCapra]

Adds the same bent normal map logic used in the mobile renderer from #89988 to the compatibility renderer.

Disabled / master	Enabled

Testing can be done with https://github.com/LunaCapra/godot-bent-normals-test

[BlueCube3310]

Enabling Bent Normal Map while Normal Map is disabled will cause a shader compilation error:

[LunaCapra]

Enabling Bent Normal Map while Normal Map is disabled will cause a shader compilation error:

I missed the forced definition of NORMAL_USED in the fragment shader, thanks for letting me know

[BlueCube3310]

Bent normal maps should probably enable the tangent/binormal shader variants as well (like in the mobile renderer), otherwise they're set to 0.

[BlueCube3310]

Looks good to me, I tested locally and it works as expected.