WebGradients: redo implementation

The previous implementation was *extremely* expensive. It
relied on loading a binary JSON file from resources (which
involved decompressing it), then extracting information out of
it to build a gradient.  Already-loaded gradients were kept in
a local cache, which had to be mutex protected.

Instead, this patch extends the gradient generator to build
static arrays filled with the web gradient data, sitting in
.rodata.

These arrays are used when building QGradient objects with a
web gradient. No explicit mutex protection is necessary, since
accesses will just read from the arrays.

As benefits, this patch removes:

* the binary json representation from QtGui's resources (~4KB
compressed, ~50KB uncompressed)
* the overhead of reading from the JSON for each used web
gradient;
* the startup costs of registering the webgradients in the
resources;
* all the overhead of mutex locking when building such
gradients;
* all the runtime memory allocations to load, parse and cache
the web gradients (including the memory + CPU spike on first
load due to the uncompression of the JSON data, as well as a
couple of deep copies).

Change-Id: If5c3d704430df76ce8faf55ee75ebd4639ba09c4
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 13 insertions, 11 deletions

diff --git a/src/gui/painting/qbrush.h b/src/gui/painting/qbrush.h
index 6a4ffab1c5..1d7199782f 100644
--- a/src/gui/painting/qbrush.h
+++ b/src/gui/painting/qbrush.h
@@ -400,16 +400,7 @@ public:
     inline bool operator!=(const QGradient &other) const
     { return !operator==(other); }
 
-private:
-    friend class QLinearGradient;
-    friend class QRadialGradient;
-    friend class QConicalGradient;
-    friend class QBrush;
-
-    Type m_type;
-    Spread m_spread;
-    QGradientStops m_stops;
-    union {
+    union QGradientData {
         struct {
             qreal x1, y1, x2, y2;
         } linear;
@@ -419,7 +410,18 @@ private:
         struct {
             qreal cx, cy, angle;
         } conical;
-    } m_data;
+    };
+
+private:
+    friend class QLinearGradient;
+    friend class QRadialGradient;
+    friend class QConicalGradient;
+    friend class QBrush;
+
+    Type m_type;
+    Spread m_spread;
+    QGradientStops m_stops;
+    QGradientData m_data;
     void *dummy; // ### Qt 6: replace with actual content (CoordinateMode, InterpolationMode, ...)
 };