Add fuzzy floating point comparison macro

Add a test macro that allows comparing two floating point values for
equality within a certain tolerance.

This macro has been independently reimplemented by various projects:

 * Clutter
 * Graphene
 * colord

#914
6 jobs for wip/rishi/issue-914 in 25 minutes and 33 seconds (queued for 11 seconds)
Status Job ID Name Coverage
  Build
passed #38587
fedora-meson-android_ndk_r16_api21_arm64

00:01:24

passed #38588
fedora-meson-mingw64

00:09:23

passed #38586
fedora-meson-x86_64

00:20:55

passed #38589
win32
msys2-mingw32

00:07:40

passed #38590
win32
vs2017-x64

00:05:42

 
  Coverage
passed #38591
coverage

00:04:35

76.6%