gpu_tonemapper: Use individual EGL contexts.

- Use individual EGL contexts for each tone-mapping
  session.
- Having one context make multiple sessions serial, this
  helps in making this parallel when possible.
- Resolves error messages which happened when the context
  was deleted during mirroring.

Change-Id: Ia8738551b4189dccffb233320a9d69fcfd7f0118
Crs-fixed: 1115057
5 files changed