VK_EXT_shader_stencil_export is a trivial extension to implement in
SwiftShader. Added PixelRoutine::stencilReplaceRef(), which can be trivially
modified to output Byte4 instead of Byte8, should the stencil code be
refactored.

Verified that these tests still pass with SwANGLE
dEQP-GLES3.functional.fbo.blit.depth_stencil.depth32f_stencil8_basic
dEQP-GLES3.functional.fbo.blit.depth_stencil.depth32f_stencil8_stencil_only
dEQP-GLES3.functional.fbo.blit.depth_stencil.depth24_stencil8_basic
dEQP-GLES3.functional.fbo.blit.depth_stencil.depth24_stencil8_stencil_only

Change-Id: I0cd8ca7310fc5a5130e05fdb546370fc7f31210b
Tests: dEQP-VK.pipeline.shader_stencil_export.op_replace
Bug: b/148175198
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40488
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>