Add Meson build definitions.

meson.build

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+project('imgui-sfml', 'cpp',
+        version: '2.1',
+        meson_version: '>=0.55.0',
+        license: 'MIT')
+
+imgui_dep = dependency('imgui', fallback: ['imgui', 'imgui_dep'])
+
+# We do not look up sfml here because there are many different
+# names for the .pc files and others and it is unclear which
+# ones we should use. Thus users of this wrap must look up sfml
+# by themselves and add it to their targets.
+#
+# This is suboptimal, hopefully this can be fixed once we have
+# more usage experience.
+
+imgui_sfml_dep = declare_dependency(include_directories: '.',
+                                    dependencies: imgui_dep)
+meson.override_dependency('imgui-sfml', imgui_sfml_dep)

upstream.wrap

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+[wrap-file]
+directory = imgui-sfml-2.1
+
+source_url = https://github.com/eliasdaler/imgui-sfml/archive/v2.1.tar.gz
+source_filename = v2.1.tar.gz
+source_hash = 16a589cb7219ebe3147b13cfe44e50de315deedf6ca8bd67d4ef91de3a09478e