VK_EXT_shader_stencil_export is a simple 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

Tests: dEQP-VK.pipeline.shader_stencil_export.op_replace
Bug: b/148175198
Change-Id: Ib98e9917a42aa487d2ee6ffed8b8e317b4fec5d3
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41208
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>