Need to support implicit conversions between types for GL shaders
Other small fixes to support GL shaders

Bug: angleproject:3673
Change-Id: I5341cb7195054ccc4cd36aad5dc8c801c7e1a14f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1698649
Commit-Queue: Clemen Deng <clemendeng@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>