Implemented switch/case for glsl in OpenGL ES 3.0.
For simplicity, it is implemented as a loop without a condition,
so break statements work properly like so:

begin switch
  if(...) // 1st case
  ...
  else if(...) // other cases
  ...
  else // default case
  ...
end switch // Anchor point for break statements

All related dEQP tests pass, except 7 tests where vertex shaders
contain a switch or a loop within another switch. These 7 failures
have only about 5% of bad pixel and seem to be related to an issue
with int(floor(...)), since the equivalent tests inside the fragment
shader pass.

KNOWN ISSUE: If a switch is within a loop and one of the cases
             contains a "continue" statement, this will not be
             handled correctly at the moment. There are no dEQP
             tests for this at the moment, AFAIK.

Change-Id: I3ba34ab06a759d07e8520f6a87d75036a5cdaef5
Reviewed-on: https://swiftshader-review.googlesource.com/5272Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>