Started moving SFML specific stuff into the sfml/ directory.

meson.build

@@ -82,9 +82,9 @@ sources = [
   'dbc.cpp',
   'guecs.cpp',
   'lel.cpp',
-  'shaders.cpp',
-  'sound.cpp',
-  'textures.cpp',
+  'sfml/shaders.cpp',
+  'sfml/sound.cpp',
+  'sfml/textures.cpp',
 ]
 
 executable('runtests', sources + [
@@ -98,3 +98,11 @@ executable('runtests', sources + [
   link_args: link_args,
   override_options: exe_defaults,
   dependencies: dependencies + [catch2])
+
+executable('calc', sources + [
+  'demos/calc.cpp',
+  ],
+  cpp_args: cpp_args,
+  link_args: link_args,
+  override_options: exe_defaults,
+  dependencies: dependencies)