Meta: Mark the other image file formats as binary in .gitattributes

We had rules for .png and .jpg files, but we have not maintained the
list as support for other file formats has been added. To test these
changes test files have been committed for each of these formats.

This change updates the list with all of the binary image file types I
was able to find in the tree at the time of writing.

.gitattributes

@@ -2,5 +2,11 @@
 * text=auto eol=lf
 
 # Denote all files that are truly binary and should not be modified.
-*.png binary
+*.bmp binary
+*.dds binary
+*.gif binary
 *.jpg binary
+*.pbm binary
+*.pgm binary
+*.png binary
+*.ppm binary