Subtitles: Make AVSubs zoom factor persistent.

Most DVD/BD subtitles are consistently sized, so it makes sense to
save the zoom factor back to the database for next time.  Since there
is no relationship between text font sizes and AVSub sizes, separate
zoom settings are maintained for text subtitles (OSDCC708TextZoom) and
AV subtitles (OSDAVSubZoom).

mythtv/libs/libmythtv/subtitlescreen.cpp

@@ -575,6 +575,7 @@ void SubtitleScreen::EnableSubtitles(int type, bool forced_only)
     ClearAllSubtitles();
     SetVisible(m_subtitleType != kDisplayNone);
     SetArea(MythRect());
+    m_textFontZoom  = gCoreContext->GetNumSetting("OSDCC708TextZoom", 100);
     switch (m_subtitleType)
     {
     case kDisplayTextSubtitle:
@@ -589,9 +590,10 @@ void SubtitleScreen::EnableSubtitles(int type, bool forced_only)
         break;
     case kDisplayAVSubtitle:
         m_family = kSubFamilyAV;
-        m_textFontZoom = m_textFontZoomPrev = 100;
+        m_textFontZoom = gCoreContext->GetNumSetting("OSDAVSubZoom", 100);
         break;
     }
+    m_textFontZoomPrev = m_textFontZoom;
 }
 
 void SubtitleScreen::DisableForcedSubtitles(void)
@@ -617,8 +619,6 @@ bool SubtitleScreen::Create(void)
         LOG(VB_GENERAL, LOG_WARNING, LOC + "Failed to get CEA-608 reader.");
     if (!m_708reader)
         LOG(VB_GENERAL, LOG_WARNING, LOC + "Failed to get CEA-708 reader.");
-    m_textFontZoom  = gCoreContext->GetNumSetting("OSDCC708TextZoom", 100);
-    m_textFontZoomPrev = m_textFontZoom;
 
     return true;
 }
@@ -1395,10 +1395,10 @@ MythFontProperties* SubtitleScreen::GetFont(CC708CharacterAttribute attr,
 void SubtitleScreen::SetZoom(int percent)
 {
     m_textFontZoom = percent;
-    if (m_family != kSubFamilyAV)
-    {
+    if (m_family == kSubFamilyAV)
+        gCoreContext->SaveSetting("OSDAVSubZoom", percent);
+    else
         gCoreContext->SaveSetting("OSDCC708TextZoom", percent);
-    }
 }
 
 int SubtitleScreen::GetZoom(void)