src: do not override explicitly requested grayscale antialiasing.

If CAIRO_ANTIALIAS_DEFAULT is selected and system is set to subpixel, it
is perfectly normal to switch to CAIRO_ANTIALIAS_SUBPIXEL.
But when the calling application specifically requested
CAIRO_ANTIALIAS_GRAY then Cairo should honor the request.

This is an issue we have had for years in GIMP, where text on images
would render differently depending on the system the file is opened on.
This is obviously not right as a graphics work should be system
independant and allow the creator to decide if one wants a text to have
grayscale or subpixel rendering (a settings which would stick when
sharing the work file). Cairo should not override this.
The CAIRO_ANTIALIAS_DEFAULT settings exists exactly for this (when we
want Cairo to choose for us, in a system-dependant way); other settings
are when we need system independance.

Thanks to Adam Fontenot for initial investigations and tests on this and
other contributors before this.
7 jobs for wip/Jehan/subpixel-antialiasing in 20 minutes and 27 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Prep
passed #6732095
fedora image

00:00:28

 
  Test
passed #6732096
fedora autotools build

00:19:59

passed #6732097
fedora meson build

00:00:34

passed #6732100
meson android arm64 fedora

00:02:07

passed #6732101
gst-macos-10.15
meson macOS

00:02:10

passed #6732098
docker windows 1809
meson vs2017 amd64

00:02:50

passed #6732099
docker windows 1809
meson vs2017 x86

00:03:05