explicitly set workspace resolver to address cargo warning

since we're using 2021 edition, resolver = 2 is the default, but this
apparently needs to be explicitly set at the workspace level

Cargo.toml

@@ -4,6 +4,7 @@ members = [
     "ggdt_imgui",
     "examples/*",
 ]
+resolver = "2"
 
 # some stuff is becoming noticably slow with default dev profile settings (opt-level = 0). especially any of the
 # fancier triangle_2d drawing stuff. while there are many optimizations still left to be made, it seems like a good