The uncovered span calculations were ignoring span groups and inverted spans.
This is now correctly handled by Tree.allSpans().

Bug: b/152192800
Change-Id: I2bd8afa3c956b03b598a3d5297cb775fd19da35d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43573
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>