Fixes to make Darwin work (#7)

`extra-libraries: sdl2` needs to be `extra-libraries: SDL2`, and we need a dependency on `OpenGL`.

Author: o1lo01ol1o

dear-imgui.cabal

@@ -56,6 +56,8 @@ library
     if os(windows)
       extra-libraries:
         opengl32
+    if os(darwin)
+      frameworks: OpenGL
     else
       extra-libraries:
         GL
@@ -70,7 +72,7 @@ library
 
     if os(windows) || os(darwin)
       extra-libraries:
-        sdl2
+        SDL2
     else
       pkgconfig-depends:
         sdl2