Add overlay and multiply effects to gdImageSetPixel
add tests for layer effects
Fix buffer overrun from long font search path

MAXPATHLEN is set to 256 (on some systems), and DEFAULT_FONTPATH is longer
than this (on some systems). The sprintf() to path[MAXPATHLEN] thus
resulted in a buffer overrun when in some circumstances (also,
fontsearchpath is in the control of the environment, so this could be
triggered on any platform).
CVE-2014-2497, NULL pointer dereference, fix #126
Merged in ttoohey/gd-libgd/bugfix/font_path_overrun (pull request #19)

Fix buffer overrun from long font search path
Merged in ttoohey/gd-libgd/layout-effect (pull request #18)

Overlay and multiply effects for gdImageSetPixel