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.

subprojects/packagefiles/cimgui/meson.build

@@ -0,0 +1,29 @@
+project('cimgui', 'cpp', version : '1.73.0', default_options: ['default_library=shared', 'warning_level=1'])
+
+# Files
+src = [
+	'cimgui.cpp',
+	'imgui/imgui.cpp',
+	'imgui/imgui_draw.cpp',
+	'imgui/imgui_demo.cpp',
+	'imgui/imgui_widgets.cpp',
+]
+
+inc = [ '.' ]
+pub_inc = [ 'imgui' ]
+
+# Dependencies
+# bindbc_loader_dep = dependency('bindbc-loader')
+
+lib = shared_library('cimgui', src,
+	# dependencies : bindbc_loader_dep,
+	include_directories : [inc, pub_inc],
+	# pic : true,
+)
+
+cimgui_dep = declare_dependency(
+	include_directories : [pub_inc],
+	link_with : lib,
+)
+
+meson.override_dependency('cimgui', cimgui_dep)