Fix out-of-bounds image slices

test/gfx/bad_slice.err

@@ -0,0 +1,3 @@
+error: Image slice ((2, 2) to (130, 130)) is outside the image bounds (20x20)!
+note: Did you mean the slice "2,2:2,2"? (width and height are in tiles, not pixels!)
+Conversion aborted after 1 error

test/gfx/bad_slice.flags

@@ -0,0 +1 @@
+-L 2,2:16,16

test/gfx/oob_slice_wh.err

@@ -0,0 +1,2 @@
+error: Image slice ((2, 2) to (34, 18)) is outside the image bounds (20x20)!
+Conversion aborted after 1 error

test/gfx/oob_slice_wh.flags

@@ -0,0 +1 @@
+-L 2,2:4,2

test/gfx/oob_slice_xy.err

@@ -0,0 +1,2 @@
+error: Image slice ((2, 99) to (10, 107)) is outside the image bounds (20x20)!
+Conversion aborted after 1 error

test/gfx/oob_slice_xy.flags

@@ -0,0 +1 @@
+-L 2,99:1,1