Align iOS hillshade functionality with Android

[DmitryAlexei]

Description

Currently, Android properly disables vertex hillshades when 3D is disabled or terrain raster hillshades are enabled, improving performance and battery life. iOS lacks this optimization.
So it is proposed to implement consistent terrain hillshades behavior across iOS and Android platforms.

ToDo

• disable vertex hillshades when 3D is off (should match Android behavior of disabling hillshading in elevation config
• disable vertex hillshade when terrain raster hillshade is on
• add possibility to switch on/off raster hillshades (slopes). Currently it is only possible to set visibility to 0

[Andrewschenko]

this invocation needs to be implemented in iOS code

[dmpr0]

There is no Hillshade functionality; only one type of colour palette for Terrain.