The scissor is now restricted to the framebuffer's size,
in order to avoid writing out of bounds of the image.

Vulkan 1.1 spec section 25.2. Scissor Test states that
"Rasterization does not produce fragments outside of the framebuffer"

Bug: b/141999739, angleproject:4060

Change-Id: Id0bce035b0ab9072f354b9f00be8d42577e4af54
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38008Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>