When building cairo with 'clang' there is an interesting warning

>subsurface.c:51:5: warning: implicit conversion from enumeration type 'enum _cairo_font_weight' to different enumeration type 'cairo_font_slant_t' (aka 'enum _cairo_font
>    CAIRO_FONT_WEIGHT_NORMAL,
>    ^~~~~~~~~~~~~~~~~~~~~~~~
>subsurface.c:52:5: warning: implicit conversion from enumeration type 'enum _cairo_font_slant' to different enumeration type 'cairo_font_weight_t' (aka 'enum _cairo_font
>    CAIRO_FONT_SLANT_NORMAL);
>    ^~~~~~~~~~~~~~~~~~~~~~~

In fact cairo_select_font_face is declared as:

http://cgit.freedesktop.org/cairo/tree/src/cairo.h#n1416

and accepts a cairo_font_slant_t followed by a cairo_font_weight_t,
but in test/subsurface.c (and test/subsurface-scale.c):

http://cgit.freedesktop.org/cairo/tree/test/subsurface.c#n49

CAIRO_FONT_SLANT_NORMAL follows CAIRO_FONT_WEIGHT_NORMAL. Given that
these tests are often used as examples or to generate more elaborate
tests I suggest to swap the arguments even if the two values are the
same (in both test files).