Add Libmypaint/Brushlib (#2587)

* Add Libmypaint/Brushlib

* Rename libmypaint.rb to libmypaint-gegl.rb

* Improved description

* Revert to libmypaint.rb

packages/libmypaint.rb

+require 'package'
+
+class Libmypaint < Package
+  description 'Libmypaint is MyPaint’s brushstroke rendering code, in a form that can be used by other programs.'
+  homepage 'http://mypaint.org/'
+  version '1.3.0'
+  source_url 'https://github.com/mypaint/libmypaint/releases/download/v1.3.0/libmypaint-1.3.0.tar.xz'
+  source_sha256 '6a07d9d57fea60f68d218a953ce91b168975a003db24de6ac01ad69dcc94a671'
+
+  depends_on 'jsonc'
+  depends_on 'gobject_introspection'
+  depends_on 'glib'
+  depends_on 'gegl'
+
+  def self.build
+    system "./configure --prefix=#{CREW_PREFIX} --libdir=#{CREW_LIB_PREFIX}"
+  end
+
+  def self.install
+    system "make", "DESTDIR=#{CREW_DEST_DIR}", "install"
+    system "ldconfig"
+  end
+end