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.

tests/meson.build

@@ -0,0 +1,13 @@
+tests_src = files(
+	'tests.d',
+)
+
+exe = executable('decs-tests',
+	tests_src,
+	include_directories : [inc],
+	d_args : args,
+	link_args : link_args,
+	dependencies : decs_dep,
+)
+
+test('basic-tests', exe)