display: Store and restore the current eglContext

Tone mapper engine setting the current eglContext and never
restored it.  With multiple eglContexts used in same thread
by SurfaceFlinger and GPU Tonemapper, it lead to incorrect
eglContexts being used, hence always store the restore the
eglContext in each GPUTonemapper call.

CRs-Fixed: 2009259
Change-Id: Ic9fe73818ddfe3881e5fa82f7853dce130bba24e
3 files changed