[Feature Request] Add a project setting to change the number of texture samplers to support in ES2

Add an item to Project Settings, Android, "Support 16 Texture Samplers on ES2". The bBuildForES2 flag must be enabled for this setting to be active.

The tooltip should read "Allow 16 texture samplers when packaging for ES2, even though the OpenGL ES 2 specification doesn't guarantee this. Devices without support will be unable to render materials using more than 8 texture sampless"

The FOpenGLFrontend::GetMaxSamplers(GLSLVersion Version) function should return 16 for GLSL_ES2 and the GetExpectedFeatureLevelMaxTextureSamplers() funciton needs to change also. Be sure to change this for the ES2 preview shader platforms also.