Fix FullscreenPreview for SimpleRenderer

The SimpleRenderer outputs unpremultiplied RGB values when we render
in a transparent background, we have to indicate that to Skia now that
we're compositing os::Surfaces (SkiaSurfaces) directly in this
FullscreenPreview command.

src/app/commands/cmd_fullscreen_preview.cpp

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2018-2022  Igara Studio S.A.
+// Copyright (C) 2018-2023  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This program is distributed under the terms of
@@ -32,6 +32,10 @@
 #include "os/surface.h"
 #include "os/system.h"
 
+#if LAF_SKIA
+  #include "os/skia/skia_surface.h"
+#endif
+
 #include <cmath>
 #include <cstring>
 
@@ -182,6 +186,19 @@ protected:
     if (m_render == nullptr) {
       m_render = os::instance()->makeRgbaSurface(m_sprite->width(),
                                                  m_sprite->height());
+
+#if LAF_SKIA
+      // The SimpleRenderer renders unpremultiplied surfaces when
+      // rendering on transparent background (this is the only place
+      // where this happens).
+      if (render.properties().outputsUnpremultiplied) {
+        // We use a little tricky with Skia indicating that the alpha
+        // is unpremultiplied
+        ((os::SkiaSurface*)m_render.get())
+          ->bitmap().setAlphaType(kUnpremul_SkAlphaType);
+      }
+#endif
+
       m_repaint = true;
     }
 
@@ -210,7 +227,10 @@ protected:
       render.renderCheckeredBackground(
         g->getInternalSurface(), m_sprite,
         gfx::Clip(g->getInternalDeltaX(),
-                  g->getInternalDeltaY(), clientBounds()));
+                  g->getInternalDeltaY(),
+                  -m_pos.x, -m_pos.y,
+                  2*g->getInternalSurface()->width(),
+                  2*g->getInternalSurface()->height()));
 
       // Invalidate the whole Graphics (as we've just modified its
       // internal os::Surface directly).

src/app/render/renderer.h

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2022  Igara Studio S.A.
+// Copyright (C) 2022-2023  Igara Studio S.A.
 //
 // This program is distributed under the terms of
 // the End-User License Agreement for Aseprite.
@@ -38,6 +38,10 @@ namespace app {
       // composite the background already painted on the screen with
       // the sprite painted in the backbuffer.
       bool requiresRgbaBackbuffer = false;
+
+      // True if renderSprite() composite an unpremultiplied RGBA
+      // surface when we draw on a transparent background.
+      bool outputsUnpremultiplied = false;
     };
 
     virtual ~Renderer() { }

src/app/render/simple_renderer.cpp

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2022  Igara Studio S.A.
+// Copyright (C) 2022-2023  Igara Studio S.A.
 //
 // This program is distributed under the terms of
 // the End-User License Agreement for Aseprite.
@@ -17,6 +17,11 @@ namespace app {
 
 using namespace doc;
 
+SimpleRenderer::SimpleRenderer()
+{
+  m_properties.outputsUnpremultiplied = true;
+}
+
 void SimpleRenderer::setRefLayersVisiblity(const bool visible)
 {
   m_render.setRefLayersVisiblity(visible);

src/app/render/simple_renderer.h

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2022  Igara Studio S.A.
+// Copyright (C) 2022-2023  Igara Studio S.A.
 //
 // This program is distributed under the terms of
 // the End-User License Agreement for Aseprite.
@@ -17,6 +17,8 @@ namespace app {
   // CPU-only.
   class SimpleRenderer : public Renderer {
   public:
+    SimpleRenderer();
+
     const Properties& properties() const override { return m_properties; }
 
     void setRefLayersVisiblity(const bool visible) override;