For OpenGL ES 3.0, the varyings limit must be increased in order
to meet the new lower limit. This doesn't solve how the varyings
count is still verified at compile time instead of link time, but
it allows us to remove an UNIMPLEMENTED() call.

Bug b/37069434

Change-Id: Ib6641352b625d56c7c3cab119fadcdc19f794a5b
Reviewed-on: https://swiftshader-review.googlesource.com/9188Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>