Does not appear to clamp timestamps in PNG files

This bug was originally reported by Thorsten Glaser (tg@mirbsd.de) in Debian bug #931039:

While comparing builds of musescore -6 and -7 I found unexpected
changes in the PNG files that are installed, they change the tIME
chunk to, apparently, the changelog time:

--- home/tg/Misc/Vendor/musescore/share/wallpaper/paper1.sng
+++ paper1.sng
@@ -6,13 +6,13 @@ IHDR {
 bKGD {red: 255;  green: 255;  blue: 255;}
 pHYs {xpixels: 3779; ypixels: 3779; per: meter;}  # (96 dpi)
 tIME {
-    # 10 Nov 2001 20:43:56 GMT
-    year:   2001
-    month:  11
-    day:    10
-    hour:   20
-    minute: 43
-    second: 56
+    # 24 Jun 2019 16:07:46 GMT
+    year:   2019
+    month:  6
+    day:    24
+    hour:   16
+    minute: 7
+    second: 46
 }
 IMAGE {
     pixels hex