Use highp to avoid float overflow in expression

float D = C * (A / B) could be relaxed to
float D =  (C * A) * (1 / B).

This is qualified in GLES Spec at chapter Evaluation of Expressions.

Note that "C * A" could exceed the maximal value of FP16.
Use highp float to keep the correct value.

Bug: 237997490
Test: Launch gallery and check thumbnail of video

Signed-off-by: Brandon Chen <brandon.chen@imgtec.com>
Change-Id: Icfa355ecb2501267b00a9615b36fb70606864b94