Fix alpha handling in Scintilla when not using wxGraphicsContext.

Fix alpha handling in Scintilla when not using wxGraphicsContext.

When using raw bitmap data access classes such as wxAlphaPixelData we must
destroy them to ensure that the changes done via them are committed to the
bitmap before drawing the bitmap.

Just add an extra block (the diff should be viewed ignoring white space to see
the only significant change) to ensure that wxAlphaPixelData is destroyed
before DrawBitmap() is called.

Closes #14680.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72525 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775