Improve meson build.

-Add all required packages as wrap dependencies
-Move source lsitnings to separate files
-Proper dependencies declarations
-Set global D compiler arguments
-Add few asserts
-Add test exe
-Other small improvements

Tested with dmd with betterC, demos executable has to be run from 'demos' directory to properly load assets.

demos/source/meson.build

@@ -0,0 +1,20 @@
+demos_src += files(
+	'app.d',
+	'demos/brick_breaker.d',
+	'demos/bullet_madnes.d',
+	'demos/particles.d',
+	'demos/physics.d',
+	'demos/sandbox.d',
+	'demos/simple.d',
+	'demos/snake.d',
+	'demos/space_invaders.d',
+	'game_core/basic.d',
+	'game_core/collision.d',
+	'game_core/job_updater.d',
+	'game_core/rendering.d',
+	'gui/component.d',
+	'gui/manager.d',
+	'gui/system.d',
+	'gui/template_.d',
+	'gui/tool_circle.d',
+)