gdiplus: Add a fallback method for getting HDC's from Graphics objects.

Native does this in more cases (at least for all bitmap graphics
objects), but using gdi32 objects when we can should perform better.

dlls/gdiplus/gdiplus_private.h

@@ -152,6 +152,12 @@ struct GpGraphics{
     UINT textcontrast; /* not used yet. get/set only */
     struct list containers;
     GraphicsContainer contid; /* last-issued container ID */
+    /* For giving the caller an HDC when we technically can't: */
+    HBITMAP temp_hbitmap;
+    int temp_hbitmap_width;
+    int temp_hbitmap_height;
+    BYTE *temp_bits;
+    HDC temp_hdc;
 };
 
 struct GpBrush{

dlls/gdiplus/graphics.c

@@ -4663,6 +4663,9 @@ GpStatus WINGDIPAPI GdipMultiplyWorldTransform(GpGraphics *graphics, GDIPCONST G
     return ret;
 }
 
+/* Color used to fill bitmaps so we can tell which parts have been drawn over by gdi32. */
+static const COLORREF DC_BACKGROUND_KEY = 0x0c0b0d;
+
 GpStatus WINGDIPAPI GdipGetDC(GpGraphics *graphics, HDC *hdc)
 {
     TRACE("(%p, %p)\n", graphics, hdc);
@@ -4673,14 +4676,61 @@ GpStatus WINGDIPAPI GdipGetDC(GpGraphics *graphics, HDC *hdc)
     if(graphics->busy)
         return ObjectBusy;
 
-    if (!graphics->hdc)
+    if (!graphics->hdc ||
+        (graphics->image && graphics->image->type == ImageTypeBitmap && ((GpBitmap*)graphics->image)->format & PixelFormatAlpha))
     {
-        WARN("no HDC for this graphics\n");
+        /* Create a fake HDC and fill it with a constant color. */
-        *hdc = NULL;
+        HDC temp_hdc;
-        return GenericError;
+        HBITMAP hbitmap;
+        GpStatus stat;
+        GpRectF bounds;
+        BITMAPINFOHEADER bmih;
+        int i;
+
+        stat = get_graphics_bounds(graphics, &bounds);
+        if (stat != Ok)
+            return stat;
+
+        graphics->temp_hbitmap_width = bounds.Width;
+        graphics->temp_hbitmap_height = bounds.Height;
+
+        bmih.biSize = sizeof(bmih);
+        bmih.biWidth = graphics->temp_hbitmap_width;
+        bmih.biHeight = -graphics->temp_hbitmap_height;
+        bmih.biPlanes = 1;
+        bmih.biBitCount = 32;
+        bmih.biCompression = BI_RGB;
+        bmih.biSizeImage = 0;
+        bmih.biXPelsPerMeter = 0;
+        bmih.biYPelsPerMeter = 0;
+        bmih.biClrUsed = 0;
+        bmih.biClrImportant = 0;
+
+        hbitmap = CreateDIBSection(NULL, (BITMAPINFO*)&bmih, DIB_RGB_COLORS,
+            (void**)&graphics->temp_bits, NULL, 0);
+        if (!hbitmap)
+            return GenericError;
+
+        temp_hdc = CreateCompatibleDC(0);
+        if (!temp_hdc)
+        {
+            DeleteObject(hbitmap);
+            return GenericError;
+        }
+
+        for (i=0; i<(graphics->temp_hbitmap_width * graphics->temp_hbitmap_height); i++)
+            ((DWORD*)graphics->temp_bits)[i] = DC_BACKGROUND_KEY;
+
+        SelectObject(temp_hdc, hbitmap);
+
+        graphics->temp_hbitmap = hbitmap;
+        *hdc = graphics->temp_hdc = temp_hdc;
+    }
+    else
+    {
+        *hdc = graphics->hdc;
     }
 
-    *hdc = graphics->hdc;
     graphics->busy = TRUE;
 
     return Ok;
@@ -4690,12 +4740,40 @@ GpStatus WINGDIPAPI GdipReleaseDC(GpGraphics *graphics, HDC hdc)
 {
     TRACE("(%p, %p)\n", graphics, hdc);
 
-    if(!graphics)
+    if(!graphics || !hdc)
         return InvalidParameter;
 
-    if(graphics->hdc != hdc || !(graphics->busy))
+    if((graphics->hdc != hdc && graphics->temp_hdc != hdc) || !(graphics->busy))
         return InvalidParameter;
 
+    if (graphics->temp_hdc == hdc)
+    {
+        DWORD* pos;
+        int i;
+
+        /* Find the pixels that have changed, and mark them as opaque. */
+        pos = (DWORD*)graphics->temp_bits;
+        for (i=0; i<(graphics->temp_hbitmap_width * graphics->temp_hbitmap_height); i++)
+        {
+            if (*pos != DC_BACKGROUND_KEY)
+            {
+                *pos |= 0xff000000;
+            }
+            pos++;
+        }
+
+        /* Write the changed pixels to the real target. */
+        alpha_blend_pixels(graphics, 0, 0, graphics->temp_bits,
+            graphics->temp_hbitmap_width, graphics->temp_hbitmap_height,
+            graphics->temp_hbitmap_width * 4);
+
+        /* Clean up. */
+        DeleteDC(graphics->temp_hdc);
+        DeleteObject(graphics->temp_hbitmap);
+        graphics->temp_hdc = NULL;
+        graphics->temp_hbitmap = NULL;
+    }
+
     graphics->busy = FALSE;
 
     return Ok;