LibGL: Better handling of texture targets and default textures

We were lacking support for default textures (i.e. calling
`glBindTexture` with a `texture` argument of `0`) which caused our
Quake2 port to render red screens whenever a video was playing. Every
texture unit is now initialized with a default 2D texture.

Additionally, we had this concept of a "currently bound target" on our
texture units which is not how OpenGL wants us to handle targets.
Calling `glBindTexture` should set the texture for the provided target
only, making it sort of an alias for future operations on the same
target.

Finally, `glDeleteTextures` should not remove the bound texture from
the target in the texture unit, but it should reset it to the default
texture.

Userland/Libraries/LibGL/CMakeLists.txt

@@ -16,7 +16,6 @@ set(SOURCES
     SoftwareGLContext.cpp
     Tex/NameAllocator.cpp
     Tex/Texture2D.cpp
-    Tex/TextureUnit.cpp
 )
 
 serenity_lib(LibGL gl)