Add material shader quality settings.
Enables quality overrides for android and ios (ES2 flavours)
#codereview jack.porter
[CL 2705914 by Allan Bentham in Main branch]
* bGenerateAmbientOcclusionMaterialMask must be enabled in WorldSettings for the mask to be produced by a lighting build. The mask is stored in a BC4 texure which adds half a byte per lightmap texel.
[CL 2608090 by Daniel Wright in Main branch]
* New light Lightmass setting 'bUseAreaShadowsForStationaryLight'
* Removed 'DistanceFieldPenumbraSize' project setting, uniform penumbra size comes from each light's LightSourceAngle or SourceRadius using a heuristic now
[CL 2590302 by Daniel Wright in Main branch]
* Diffuse lighting on the opposite side of the leaf becomes subsurface lighting on the side being shaded. SubsurfaceColor filters this lighting and can be used to represent varying thickness as well (leaf veins). Subsurface lighting is a mix of wrap-around diffuse with a directionally dependent lobe.
* Opacity only affects shadow falloff distance
[CL 2353634 by Daniel Wright in Main branch]
* Materials created to use the max samplers of D3D11 will simply fail to compile on OpenGL 4, so the project gets to choose if they care about GL 4
[CL 2340182 by Daniel Wright in Main branch]
