Make max canvas scale 1600% (fix #15861)

Max canvas sacle should be a multiple of 2 so that zooming in
to the max and then back out again results in 100% zoom option.

Additionally, make the min/max zoom level not come from QSettings
as these aren't exposed anywhere

src/app/qgsoptions.cpp

@@ -600,8 +600,8 @@ QgsOptions::QgsOptions( QWidget *parent, Qt::WindowFlags fl )
   mSimplifyMaximumScaleComboBox->setScale( 1.0 / mSettings->value( QStringLiteral( "/qgis/simplifyMaxScale" ), 1 ).toFloat() );
 
   // Magnifier
-  double magnifierMin = 100 * mSettings->value( QStringLiteral( "/qgis/magnifier_factor_min" ), 0.1 ).toDouble();
-  double magnifierMax = 100 * mSettings->value( QStringLiteral( "/qgis/magnifier_factor_max" ), 10 ).toDouble();
+  double magnifierMin = 100 * QgisGui::CANVAS_MAGNIFICATION_MIN;
+  double magnifierMax = 100 * QgisGui::CANVAS_MAGNIFICATION_MAX;
   double magnifierVal = 100 * mSettings->value( QStringLiteral( "/qgis/magnifier_factor_default" ), 1.0 ).toDouble();
   doubleSpinBoxMagnifierDefault->setRange( magnifierMin, magnifierMax );
   doubleSpinBoxMagnifierDefault->setSingleStep( 50 );

src/app/qgsstatusbarmagnifierwidget.cpp

@@ -22,14 +22,15 @@
 #include <qgsapplication.h>
 #include "qgsstatusbarmagnifierwidget.h"
 #include "qgsdoublespinbox.h"
+#include "qgisgui.h"
 
 QgsStatusBarMagnifierWidget::QgsStatusBarMagnifierWidget( QWidget* parent )
     : QWidget( parent )
 {
   QSettings settings;
-  int minimumFactor = ( int ) 100 * settings.value( QStringLiteral( "/qgis/magnifier_factor_min" ), 0.1 ).toDouble();
-  int maximumFactor = ( int ) 100 * settings.value( QStringLiteral( "/qgis/magnifier_factor_max" ), 10 ).toDouble();
-  int defaultFactor = ( int ) 100 * settings.value( QStringLiteral( "/qgis/magnifier_factor_default" ), 1.0 ).toDouble();
+  int minimumFactor = 100 * QgisGui::CANVAS_MAGNIFICATION_MIN;
+  int maximumFactor = 100 * QgisGui::CANVAS_MAGNIFICATION_MAX;
+  int defaultFactor = 100 * settings.value( QStringLiteral( "/qgis/magnifier_factor_default" ), 1.0 ).toDouble();
 
   // label
   mLabel = new QLabel();

src/gui/qgisgui.h

@@ -25,6 +25,7 @@ class QFont;
 /** \ingroup gui
  * /namespace QgisGui
  * The QgisGui namespace contains constants and helper functions used throughout the QGIS GUI.
+ * \note not available in Python bindings
  */
 namespace QgisGui
 {
@@ -48,6 +49,21 @@ namespace QgisGui
    */
   static const Qt::WindowFlags ModalDialogFlags = 0;
 
+  /**
+   * Minimum magnification level allowed in map canvases.
+   * @see CANVAS_MAGNIFICATION_MAX
+   * @note added in QGIS 3.0
+   */
+  constexpr double CANVAS_MAGNIFICATION_MIN = 0.1;
+
+  /**
+   * Maximum magnification level allowed in map canvases.
+   * @see CANVAS_MAGNIFICATION_MAX
+   * @note added in QGIS 3.0
+   */
+  // Must be a factor of 2, so zooming in to max from 100% then zooming back out will result in 100% mag
+  constexpr double CANVAS_MAGNIFICATION_MAX = 16.0;
+
   /**
     Open files, preferring to have the default file selector be the
     last one used, if any; also, prefer to start in the last directory

src/gui/qgsmapcanvas.cpp

@@ -225,9 +225,8 @@ QgsMapCanvas::~QgsMapCanvas()
 void QgsMapCanvas::setMagnificationFactor( double factor )
 {
   // do not go higher or lower than min max magnification ratio
-  QSettings settings;
-  double magnifierMin = settings.value( QStringLiteral( "/qgis/magnifier_factor_min" ), 0.1 ).toDouble();
-  double magnifierMax = settings.value( QStringLiteral( "/qgis/magnifier_factor_max" ), 10 ).toDouble();
+  double magnifierMin = QgisGui::CANVAS_MAGNIFICATION_MIN;
+  double magnifierMax = QgisGui::CANVAS_MAGNIFICATION_MAX;
   factor = qBound( magnifierMin, factor, magnifierMax );
 
   // the magnifier widget is in integer percent