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.

(cherry picked from commit 05dd2fa298)

docs/news.dox

@@ -299,6 +299,13 @@ a requirement of the GLFW CMake files.  The GLFW source files do not depend on
 CMake.
 
 
+@subsubsection caveat_fbtransparency_33 Framebuffer transparency requires DWM transparency
+
+GLFW no longer supports framebuffer transparency enabled via @ref
+GLFW_TRANSPARENT_FRAMEBUFFER on Windows 7 if DWM transparency is off
+(the Transparency setting under Personalization > Window Color).
+
+
 @subsection deprecations_33 Deprecations in version 3.3
 
 @subsubsection charmods_callback_33 Character with modifiers callback

docs/window.dox

@@ -234,13 +234,6 @@ alpha channel will be used to combine the framebuffer with the background.  This
 does not affect window decorations.  Possible values are `GLFW_TRUE` and
 `GLFW_FALSE`.
 
-@par
-@win32 GLFW sets a color key for the window to work around repainting issues
-with a transparent framebuffer.  The chosen color value is RGB 255,0,255
-(magenta).  This will make pixels with that exact color fully transparent
-regardless of their alpha values.  If this is a problem, make these pixels any
-other color before buffer swap.
-
 @anchor GLFW_FOCUS_ON_SHOW_hint
 __GLFW_FOCUS_ON_SHOW__ specifies whether the window will be given input
 focus when @ref glfwShowWindow is called. Possible values are `GLFW_TRUE` and `GLFW_FALSE`.