Win32: Disable fb transparency when it is broken

On Windows 7, when GLFW framebuffer transparency and the DWM are enabled
but DWM transparency is disabled (i.e. when the Transparency setting is
disabled under Personalization > Color), the contents of the framebuffer
is combined with the last frame using additive blending instead of
replacing the previous contents.

This commit limits GLFW framebuffer transparency on Windows 7 to when
DWM transparency is enabled, removing the previous workaround of setting
a layered window color key that led to rendering artifacts.

Fixes #1512.

src/win32_init.c

@@ -143,6 +143,8 @@ static GLFWbool loadLibraries(void)
             GetProcAddress(_glfw.win32.dwmapi.instance, "DwmFlush");
         _glfw.win32.dwmapi.EnableBlurBehindWindow = (PFN_DwmEnableBlurBehindWindow)
             GetProcAddress(_glfw.win32.dwmapi.instance, "DwmEnableBlurBehindWindow");
+        _glfw.win32.dwmapi.GetColorizationColor = (PFN_DwmGetColorizationColor)
+            GetProcAddress(_glfw.win32.dwmapi.instance, "DwmGetColorizationColor");
     }
 
     _glfw.win32.shcore.instance = LoadLibraryA("shcore.dll");