Updated copyright year for the project as a whole.

Source/DFPSR/License.txt

@@ -1,7 +1,7 @@
 Main license for David Piuva's software renderer:
 	zlib open source license
 
-	Copyright (c) 2017 to 2019 David Forsgren Piuva
+	Copyright (c) 2017 to 2021 David Forsgren Piuva
 
 	This software is provided 'as-is', without any express or implied
 	warranty. In no event will the authors be held liable for any damages
@@ -24,4 +24,3 @@ Main license for David Piuva's software renderer:
 
 External licenses:
 	* See image/stbImage for STB's license and copyright notice.
-