UMG element clipping does not work on Galaxy S8 (Mali)

UMG clipping requires stencil.
Seems like we don't request stencil when creating backbuffer

Changing
const int EGLMinStencilBits = 0;
to
const int EGLMinStencilBits = 8;

fixes this.

Deploy attached project to S8 with Mali GPU
Compare UI rendering to what is rendered in PIE