[cairo] Rel. 1.12 - quartz surface on IOS

[cu]

First, thank you for updating Quartz surface to use "blend mode" and
removing references to "SetCTM".

- Blend modes are turned on based on compile-time define
__MAC_OS_X_VERSION_MIN_REQUIRED
  This is not available on IOS. Blend modes are available on all
versions of IOS (and, on the other hand, using private composite mode
API is against appstore rules)
  Would be great if the condition was updated to include IOS (i.e. #if
__MAC_OS_X_VERSION_MIN_REQUIRED || _IOS_WHATEVER_ )

- To build Quartz surface on IOS, includes in cairo-quartz.h need to be
changed to:
#include <QuartzCore/QuartzCore.h>
#include <CoreGraphics/CoreGraphics.h>
#include <CoreGraphics/CGContext.h>

- There are two test functions to read/write Quartz bitmap surface to
PNG file. I wonder if it may be possible to expose those through the
official API?

While it's not a big deal to edit a few files before building for IOS,
having this in official source code would definitely help.