Refactor discard/clear behavior in DirectRenderer

This CL tries to simplify the logic around per-pass scissor set-up.
- For a given pass, we calculate the intersection of the viewport rect,
  the clip rect and any scissor rect.
- We check whether this intersection contains the entire output surface
  for the pass - if not, we consider the pass "clipped"
- If the pass is clipped we disable GL discard and restrict clear to
  the clipped region. We also clip all quads to the clipped region
  (as before).
- If the pass is not clipped, we perform a discard/clear and render
  all quads as normal (no clipping).

This flow seems more straightforward, as we don't need to track as
many booleans/values and treat things more uniformly. It does result
in slightly different behavior which required a few unit test changes
(not pixel tests, but tests that expected certain patterns of
scissors/etc...):
- If we are using the scissor optimization, but the scissor rect
  contains the entire output surface, we proceed as though we aren't
  using the scissor optimization (previously we would apply
  full-output-surface clips).
- If both device and scissor clips are applied, we use the intersection
  of these to restrict clear (before we would just use the device clip).
- If the viewport does not contain the full output surface, the clear is
  restricted to the viewport (before it would clear the entire surface).

BUG=468845

Review URL: https://codereview.chromium.org/1018423002

Cr-Commit-Position: refs/heads/master@{#321697}

1 files changed, 20 insertions, 9 deletions

diff --git a/cc/output/software_renderer.cc b/cc/output/software_renderer.cc
index cecd633..bd99d0b1 100644
--- a/cc/output/software_renderer.cc
+++ b/cc/output/software_renderer.cc
@@ -172,10 +172,6 @@ bool SoftwareRenderer::BindFramebufferToTexture(
   current_framebuffer_canvas_ =
       skia::AdoptRef(new SkCanvas(current_framebuffer_lock_->sk_bitmap()));
   current_canvas_ = current_framebuffer_canvas_.get();
-  InitializeViewport(frame,
-                     target_rect,
-                     gfx::Rect(target_rect.size()),
-                     target_rect.size());
   return true;
 }
 
@@ -202,11 +198,7 @@ void SoftwareRenderer::ClearCanvas(SkColor color) {
     current_canvas_->clear(color);
 }
 
-void SoftwareRenderer::DiscardPixels(bool has_external_stencil_test,
-                                     bool draw_rect_covers_full_surface) {}
-
-void SoftwareRenderer::ClearFramebuffer(DrawingFrame* frame,
-                                        bool has_external_stencil_test) {
+void SoftwareRenderer::ClearFramebuffer(DrawingFrame* frame) {
   if (frame->current_render_pass->has_transparent_background) {
     ClearCanvas(SkColorSetARGB(0, 0, 0, 0));
   } else {
@@ -218,6 +210,25 @@ void SoftwareRenderer::ClearFramebuffer(DrawingFrame* frame,
   }
 }
 
+void SoftwareRenderer::PrepareSurfaceForPass(
+    DrawingFrame* frame,
+    SurfaceInitializationMode initialization_mode,
+    const gfx::Rect& render_pass_scissor) {
+  switch (initialization_mode) {
+    case SURFACE_INITIALIZATION_MODE_PRESERVE:
+      EnsureScissorTestDisabled();
+      return;
+    case SURFACE_INITIALIZATION_MODE_FULL_SURFACE_CLEAR:
+      EnsureScissorTestDisabled();
+      ClearFramebuffer(frame);
+      break;
+    case SURFACE_INITIALIZATION_MODE_SCISSORED_CLEAR:
+      SetScissorTestRect(render_pass_scissor);
+      ClearFramebuffer(frame);
+      break;
+  }
+}
+
 void SoftwareRenderer::SetDrawViewport(
     const gfx::Rect& window_space_viewport) {}