[textures] Fix LoadTextureCubemap for manual layouts

[Not-Nik]

Passing any other layout than CUBEMAP_LAYOUT_AUTO_DETECT to LoadTextureCubemap loads an empty texture with 0 by 0 dimensions. This is because the size of each face is caculated like so

int size = cubemap.width;

but cubemap is zero initialized and only set when auto detecting the layout.

This PR fixes this issue, by manually setting the width for each layout.

[raysan5]

@Not-Nik thanks for the review! Actually I think that function had some other issues... and an example was required! I think it was not properly tested...

[Not-Nik]

There is an example for it, and from what I've seen it does work for the layouts it actually has implementations for.

It only isn't implemented for CUBEMAP_LAYOUT_PANORAMA, but the aforementioned example does have code that loads those.

[raysan5]

@Not-Nik Wow! Really? I didn't remember about it! 😄