Always blend the border pixels with borderColor.
Bounce off the border: gfedcb|abcdefgh|gfedcb.
Blend the border pixels with surrounding pixels, if possible, or with borderColor if the border is at the edge of the source image.
Mirror the last few border pixels: fedcba|abcdefgh|hgfedcb.
Repeat the very border pixel color: aaaaaa|abcdefgh|hhhhhh.
Take colors from the opposite edge: cdefgh|abcdefgh|abcdef.
The mapping mode for the pixels outside of the border in the Gaussian blur effect.