RenderingDevice: Fix getting cubemap layer data

[BlueCube3310]

I've noticed this while working on #94600.

The code for retrieving cubemap layer data would multiply the layer count by 6, even though each cube face was already considered a separate layer. Due to this, the DEV_ASSERT below would get triggered due to mismatched data sizes.

[akien-mga]

Thanks!

[clayjohn]

This might need more careful testing, the same code is in texture_update and texture_copy. In all cases they are reading from the tex->layers property which is set at creation time

[BlueCube3310]

After looking at the code, the layer_count variable appears to only be used for ensuring the provided index doesn't reach outside of the layers array.

[clayjohn]

After looking at the code, the layer_count variable appears to only be used for ensuring the provided index doesn't reach outside of the layers array.

Very interesting. So it should be easy to validate then. I imagine the fix you did here would apply in those places too.