[RenderEngine] Clamp input color for BT2020_PQ EOTF

Color input to EOTF for PQ transfer function is out-of-bounds [0.0, 1.0]
Reason being, texture2D returns values which is not in range [0.0, 1.0].
These returned values are passed as gl_FragColor.rgb to EOTF for PQ
transfer function. In EOTF, colors outside the range [0.0, 1.0] are
rendered as black resulting in artifacts while video playback.
Clamp the input color to the range [0.0, 1.0]

Bug: 118794307
Test: Youtube HDR video plays fine, also screenshots taken while playback
  does not have any artifacts.

Change-Id: I266f5a009c4c58924510283295757abae811a41e