gtkcssimagelinear.c: Avoid VLAs

gtkcssimagelinear.c: Avoid VLAs

VLAs are not supported by Visual Studio and possibly other compilers that
are supported by GTK+-3.90+, and probably never will be, although it is a
C99 specification, and it became optional for C11.  It is also not a part
of the newer compiler requirements that are listed out for GTK+-3.90.x.

There exist concerns about the implementation of VLAs in compilers that
support them as well, so change it to a g_newa() approach.

https://bugzilla.gnome.org/show_bug.cgi?id=773299