metainfo.xml: Use 1600x1200px screenshots

Fixes the following appstream-util validation errors:

    • attribute-invalid     : <screenshot> width too large [https://mixxx.org/theme/images/2.3/screenshots/latenight-palemoon-3840x2160.png] maximum is 1600px
    • attribute-invalid     : <screenshot> height too large [https://mixxx.org/theme/images/2.3/screenshots/latenight-palemoon-3840x2160.png] maximum is 900px
    • attribute-invalid     : <screenshot> width too large [https://mixxx.org/theme/images/2.3/screenshots/deere-3840x2160.png] maximum is 1600px
    • attribute-invalid     : <screenshot> height too large [https://mixxx.org/theme/images/2.3/screenshots/deere-3840x2160.png] maximum is 900px
    • attribute-invalid     : <screenshot> width too large [https://mixxx.org/theme/images/2.3/screenshots/latenight-classic-3840x2160.png] maximum is 1600px
    • attribute-invalid     : <screenshot> height too large [https://mixxx.org/theme/images/2.3/screenshots/latenight-classic-3840x2160.png] maximum is 900px
    • attribute-invalid     : <screenshot> width too large [https://mixxx.org/theme/images/2.3/screenshots/shade-classic-3840x2160.png] maximum is 1600px
    • attribute-invalid     : <screenshot> height too large [https://mixxx.org/theme/images/2.3/screenshots/shade-classic-3840x2160.png] maximum is 900px
    • attribute-invalid     : <screenshot> width too large [https://mixxx.org/theme/images/2.3/screenshots/shade-dark-3840x2160.png] maximum is 1600px
    • attribute-invalid     : <screenshot> height too large [https://mixxx.org/theme/images/2.3/screenshots/shade-dark-3840x2160.png] maximum is 900px
    • attribute-invalid     : <screenshot> width too large [https://mixxx.org/theme/images/2.3/screenshots/shade-summersunset-3840x2160.png] maximum is 1600px
    • attribute-invalid     : <screenshot> height too large [https://mixxx.org/theme/images/2.3/screenshots/shade-summersunset-3840x2160.png] maximum is 900px
    • attribute-invalid     : <screenshot> width too large [https://mixxx.org/theme/images/2.3/screenshots/tango-3840x2160.png] maximum is 1600px
    • attribute-invalid     : <screenshot> height too large [https://mixxx.org/theme/images/2.3/screenshots/tango-3840x2160.png] maximum is 900px

res/linux/org.mixxx.Mixxx.metainfo.xml

@@ -59,31 +59,31 @@
   </provides>
   <screenshots>
     <screenshot type="default">
-      <image>https://mixxx.org/theme/images/2.3/screenshots/latenight-palemoon-3840x2160.png</image>
+      <image>https://mixxx.org/theme/images/2.3/screenshots/latenight-palemoon-1600x1200.png</image>
       <caption>Mixxx with the default LateNight PaleMoon skin</caption>
     </screenshot>
     <screenshot>
-      <image>https://mixxx.org/theme/images/2.3/screenshots/latenight-classic-3840x2160.png</image>
+      <image>https://mixxx.org/theme/images/2.3/screenshots/latenight-classic-1600x1200.png</image>
       <caption>Mixxx with the LateNight Classic skin</caption>
     </screenshot>
     <screenshot>
-      <image>https://mixxx.org/theme/images/2.3/screenshots/deere-3840x2160.png</image>
+      <image>https://mixxx.org/theme/images/2.3/screenshots/deere-1600x1200.png</image>
       <caption>Mixxx with the Deere skin</caption>
     </screenshot>
     <screenshot>
-      <image>https://mixxx.org/theme/images/2.3/screenshots/shade-classic-3840x2160.png</image>
+      <image>https://mixxx.org/theme/images/2.3/screenshots/shade-classic-1600x1200.png</image>
       <caption>Mixxx with the Shade Classic skin</caption>
     </screenshot>
     <screenshot>
-      <image>https://mixxx.org/theme/images/2.3/screenshots/shade-dark-3840x2160.png</image>
+      <image>https://mixxx.org/theme/images/2.3/screenshots/shade-dark-1600x1200.png</image>
       <caption>Mixxx with the Shade Dark skin</caption>
     </screenshot>
     <screenshot>
-      <image>https://mixxx.org/theme/images/2.3/screenshots/shade-summersunset-3840x2160.png</image>
+      <image>https://mixxx.org/theme/images/2.3/screenshots/shade-summersunset-1600x1200.png</image>
       <caption>Mixxx with the Shade Summer Sunset skin</caption>
     </screenshot>
     <screenshot>
-      <image>https://mixxx.org/theme/images/2.3/screenshots/tango-3840x2160.png</image>
+      <image>https://mixxx.org/theme/images/2.3/screenshots/tango-1600x1200.png</image>
       <caption>Mixxx with the Tango skin</caption>
     </screenshot>
   </screenshots>