Add corrupted PNGs to check error handling

This is not intended to test libpng as much as checking that
we behave correctly if libpng gives us an error

test/gfx/damaged1.err

@@ -0,0 +1,2 @@
+FATAL: Error reading input image ("damaged1.png"): IDAT: invalid code -- missing end-of-block
+Conversion aborted after 1 error

test/gfx/damaged2.err

@@ -0,0 +1,2 @@
+FATAL: Error reading input image ("damaged2.png"): IDAT: invalid code -- missing end-of-block
+Conversion aborted after 1 error

test/gfx/damaged9.err

@@ -0,0 +1,2 @@
+FATAL: Error reading input image ("damaged9.png"): IDAT: invalid code -- missing end-of-block
+Conversion aborted after 1 error