- Use internalformat parameter if valid, instead of deriving from
  format/type parameters.
- Validate format/type/internalformat parameters in CopyTexSubImage().
- Moved early-out optimizations after validation.
- Removed duplicate validation.
- Use GLint consistently for internalformat parameters.

Change-Id: I377c6bb5381602d13d281f19985aa4f11d201099
Reviewed-on: https://swiftshader-review.googlesource.com/14488Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>