3d/terrain: Improve debugging tile info visibility

The `show map tile info` option allows to display the tile information
of the terrain. However this info is not really visible because the
info is in the same color as the scene background (white) and the text
is too small.

This issue is fixed with the following changes:
- change the color to red
- make the text background opaque
- increase the text size

src/3d/terrain/qgsterraintexturegenerator_p.cpp

@@ -109,7 +109,11 @@ void QgsTerrainTextureGenerator::waitForFinished()
     {
       // extra tile information for debugging
       QPainter p( &img );
-      p.setPen( Qt::white );
+      p.setPen( Qt::red );
+      p.setBackgroundMode( Qt::OpaqueMode );
+      QFont font = p.font();
+      font.setPixelSize( std::max( 30, mMap.mapTileResolution() / 6 ) );
+      p.setFont( font );
       p.drawRect( 0, 0, img.width() - 1, img.height() - 1 );
       p.drawText( img.rect(), jobData.debugText, QTextOption( Qt::AlignCenter ) );
       p.end();
@@ -139,7 +143,11 @@ void QgsTerrainTextureGenerator::onRenderingFinished()
   {
     // extra tile information for debugging
     QPainter p( &img );
-    p.setPen( Qt::white );
+    p.setPen( Qt::red );
+    p.setBackgroundMode( Qt::OpaqueMode );
+    QFont font = p.font();
+    font.setPixelSize( std::max( 30, mMap.mapTileResolution() / 6 ) );
+    p.setFont( font );
     p.drawRect( 0, 0, img.width() - 1, img.height() - 1 );
     p.drawText( img.rect(), jobData.debugText, QTextOption( Qt::AlignCenter ) );
     p.end();