gl_shader_cache: Fix texture view for cubemaps as cubemap arrays

Cubemaps are considered layered and to create a texture view the texture
mustn't be a layered texture, resulting in cubemaps being bound as
cubemap arrays. To fix this issue this commit introduces an extra
surface parameter called "is_array" and uses this to query for texture
view creation.

Now that texture views for cubemaps are actually being created, this
also fixes the number of layers created for the texture view (since they
have to be 6 to create a texture view of cubemaps).