Adding switch case default keywords

This adds switch/case keywords to glsl.
This makes the shaders that include these
keywords compile, but they don't run
properly yet, partly because the "break"
statement only handles loops currently.

Change-Id: I1f52b445e1124ed3931843c46e27ecc1d9ce3d11
Reviewed-on: https://swiftshader-review.googlesource.com/3400
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>