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.

.gitignore

@@ -8,6 +8,7 @@
 !codecov.yml
 !skeleton.html
 !**/meson.build
+!**/*.wrap
 !meson_options.txt
 !compile_wasm.py
 !compile_android.py