win32: transparent canvases

original commit: a6d25247283f5aef4287b78018f0cafb502f7559

collects/mred/private/wx/win32/canvas.rkt

@@ -31,6 +31,7 @@
 (define _HRGN _pointer)
 (define-user32 GetDCEx (_wfun _HWND _HRGN _DWORD -> _HDC))
 (define DCX_WINDOW           #x00000001)
+(define DCX_CACHE            #x00000002)
 
 (define EP_EDITTEXT 1)
 (define ETS_NORMAL 1)
@@ -151,7 +152,7 @@
          (if control-border-theme
              (let* ([r (GetWindowRect canvas-hwnd)]
                     [res (default w msg wParam lParam)]
-                    [hdc (GetDCEx canvas-hwnd #f DCX_WINDOW)]
+                    [hdc (GetDCEx canvas-hwnd #f (bitwise-ior DCX_CACHE DCX_WINDOW))]
                     [wr (make-RECT 0 0
                                    (- (RECT-right r) (RECT-left r))
                                    (- (RECT-bottom r) (RECT-top r)))])

collects/mred/private/wx/win32/wndclass.rkt

@@ -100,16 +100,14 @@
 				     #f ; menu
 				     "PLTFrame")))
 
-(void (RegisterClassW (make-WNDCLASS CS_OWNDC
+(void (RegisterClassW (make-WNDCLASS 0 ; using CS_OWNDC creates trouble when resizing?
 				     wind-proc
 				     0
                                      0
 				     hInstance
 				     #f
                                      #f
-                                     (let ([p (ptr-add #f (+ COLOR_WINDOW 1))])
-                                       (cpointer-push-tag! p 'HBRUSH)
-                                       p)
+                                     #f ; transparent
 				     #f ; menu
 				     "PLTCanvas")))