Qt/spinner: reduce cpu usage from 12% to 9% (#21552)

* faster spinner

* reduce to 6%

* Revert "reduce to 6%"

This reverts commit 65e23bb7a9542ccca13b8f01b7a5d56580a61bf6.
old-commit-hash: 776516d72b

selfdrive/ui/qt/spinner.cc

@@ -15,20 +15,24 @@
 #include "selfdrive/ui/qt/util.h"
 
 TrackWidget::TrackWidget(QWidget *parent) : QWidget(parent) {
+  setAttribute(Qt::WA_OpaquePaintEvent);
   setFixedSize(spinner_size);
-  setAutoFillBackground(true);
-  setPalette(Qt::black);
 
   // pre-compute all the track imgs. make this a gif instead?
   QPixmap comma_img = QPixmap("../assets/img_spinner_comma.png").scaled(spinner_size, Qt::KeepAspectRatio, Qt::SmoothTransformation);
-  QTransform transform(1, 0, 0, 1, width() / 2, height() / 2);
   QPixmap track_img = QPixmap("../assets/img_spinner_track.png").scaled(spinner_size, Qt::KeepAspectRatio, Qt::SmoothTransformation);
-  for (QPixmap &img : track_imgs) {
-    img = comma_img;
-    QPainter p(&img);
+
+  QTransform transform(1, 0, 0, 1, width() / 2, height() / 2);
+  QPixmap pm(spinner_size);
+  QPainter p(&pm);
   p.setRenderHint(QPainter::SmoothPixmapTransform);
+  for (int i = 0; i < track_imgs.size(); ++i) {
+    p.resetTransform();
+    p.fillRect(0, 0, spinner_size.width(), spinner_size.height(), Qt::black);
+    p.drawPixmap(0, 0, comma_img);
     p.setTransform(transform.rotate(360 / spinner_fps));
     p.drawPixmap(-width() / 2, -height() / 2, track_img);
+    track_imgs[i] = pm.copy();
   }
 
   m_anim.setDuration(1000);