Remove support for .pkm, .ktx, .astc, .dng, .heif, and .heic formats

While the skia library claimed these should all be enabled, I've been
unable to actually load those types of image files, so have removed the
illusion of support for them. There are Go implementations for some of
these that could be used instead, albeit likely slower, as they won't
get the benefit of various compiler optimizations that can be done in C.

image_format.go

@@ -27,11 +27,16 @@ const (
 	PNG
 	WBMP
 	WEBP
-	PKM
-	KTX
-	ASTC
-	DNG
-	HEIF
+	/*
+		The following formats, though supported in theory, fail to work with the cskia builds I've done.
+
+		PKM
+		KTX
+		ASTC
+		DNG
+		HEIF
+	*/
+	knownEncodedImageFormatCount
 	UnknownEncodedImageFormat EncodedImageFormat = 255
 )
 
@@ -43,11 +48,15 @@ var imageFormatToExtensions = map[EncodedImageFormat][]string{
 	PNG:  {".png"},
 	WBMP: {".wbmp"},
 	WEBP: {".webp"},
-	PKM:  {".pkm"},
-	KTX:  {".ktx"},
-	ASTC: {".astc"},
-	DNG:  {".dng"},
-	HEIF: {".heif", ".heic"},
+	/*
+		The following formats, though supported in theory, fail to work with the cskia builds I've done.
+
+		PKM:  {".pkm"},
+		KTX:  {".ktx"},
+		ASTC: {".astc"},
+		DNG:  {".dng"},
+		HEIF: {".heif", ".heic"},
+	*/
 }
 
 var (
@@ -74,7 +83,7 @@ func (e EncodedImageFormat) String() string {
 
 // CanRead returns true if the format can be read.
 func (e EncodedImageFormat) CanRead() bool {
-	return e <= HEIF
+	return e < knownEncodedImageFormatCount
 }
 
 // CanWrite returns true if the format can be written.