sw_engine: properly update the render region

shape render region should be aligned with the
current shape bbox. this could bring the compact size
of the rendering region even shapes are not drawable.

src/renderer/sw_engine/tvgSwMath.cpp

@@ -303,9 +303,7 @@ bool mathUpdateOutlineBBox(const SwOutline* outline, const SwBBox& clipRegion, S
         if (yMin > pt->y) yMin = pt->y;
         if (yMax < pt->y) yMax = pt->y;
     }
-    //Since no antialiasing is applied in the Fast Track case,
+
-    //the rasterization region has to be rearranged.
-    //https://github.com/Samsung/thorvg/issues/916
     if (fastTrack) {
         renderRegion.min.x = static_cast<SwCoord>(nearbyint(xMin / 64.0f));
         renderRegion.max.x = static_cast<SwCoord>(nearbyint(xMax / 64.0f));

src/renderer/sw_engine/tvgSwRenderer.cpp

@@ -41,7 +41,7 @@ struct SwTask : Task
 {
     SwSurface* surface = nullptr;
     SwMpool* mpool = nullptr;
-    SwBBox bbox = {{0, 0}, {0, 0}};       //Whole Rendering Region
+    SwBBox bbox;                          //Rendering Region
     Matrix transform;
     Array<RenderData> clips;
     RenderUpdateFlag flags = RenderUpdateFlag::None;
@@ -112,11 +112,14 @@ struct SwShapeTask : SwTask
 
     void run(unsigned tid) override
     {
-        if (opacity == 0 && !clipper) return;  //Invisible
+        //Invisible
+        if (opacity == 0 && !clipper) {
+            bbox.reset();
+            return;
+        }
 
         auto strokeWidth = validStrokeWidth();
         bool visibleFill = false;
-        auto clipRegion = bbox;
 
         //This checks also for the case, if the invisible shape turned to visible by alpha.
         auto prepareShape = false;
@@ -128,10 +131,11 @@ struct SwShapeTask : SwTask
             rshape->fillColor(nullptr, nullptr, nullptr, &alpha);
             alpha = MULTIPLY(alpha, opacity);
             visibleFill = (alpha > 0 || rshape->fill);
-            if (visibleFill || clipper) {
             shapeReset(&shape);
-                if (!shapePrepare(&shape, rshape, transform, clipRegion, bbox, mpool, tid, clips.count > 0 ? true : false)) {
+            if (visibleFill || clipper) {
+                if (!shapePrepare(&shape, rshape, transform, bbox, shape.bbox, mpool, tid, clips.count > 0 ? true : false)) {
                     visibleFill = false;
+                    shape.bbox.reset();
                 }
             }
         }
@@ -152,7 +156,7 @@ struct SwShapeTask : SwTask
         if (flags & (RenderUpdateFlag::Path | RenderUpdateFlag::Stroke | RenderUpdateFlag::Transform)) {
             if (strokeWidth > 0.0f) {
                 shapeResetStroke(&shape, rshape, transform);
-                if (!shapeGenStrokeRle(&shape, rshape, transform, clipRegion, bbox, mpool, tid)) goto err;
+                if (!shapeGenStrokeRle(&shape, rshape, transform, bbox, shape.bbox, mpool, tid)) goto err;
 
                 if (auto fill = rshape->strokeFill()) {
                     auto ctable = (flags & RenderUpdateFlag::GradientStroke) ? true : false;
@@ -177,9 +181,13 @@ struct SwShapeTask : SwTask
             //Clip stroke rle
             if (shape.strokeRle && !clipper->clip(shape.strokeRle)) goto err;
         }
+
+        bbox = shape.bbox; //sync
+
         return;
 
     err:
+        bbox.reset();
         shapeReset(&shape);
         shapeDelOutline(&shape, mpool, tid);
     }

src/renderer/sw_engine/tvgSwShape.cpp

@@ -498,9 +498,6 @@ bool shapePrepare(SwShape* shape, const RenderShape* rshape, const Matrix& trans
     if (!_genOutline(shape, rshape, transform, mpool, tid, hasComposite)) return false;
     if (!mathUpdateOutlineBBox(shape->outline, clipRegion, renderRegion, shape->fastTrack)) return false;
 
-    //Keep it for Rasterization Region
-    shape->bbox = renderRegion;
-
     //Check valid region
     if (renderRegion.max.x - renderRegion.min.x < 1 && renderRegion.max.y - renderRegion.min.y < 1) return false;