Respect loadOp when clearing attachments in beginRenderPass().

Bug b/119621736

Change-Id: Id9ad0f1ad9dc6116264782be3370c37c078d6846
Reviewed-on: https://swiftshader-review.googlesource.com/c/24050Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>