Fix crash accessing 24bpp image data with specific row sizes

Fixes aseprite/aseprite#2979

get_data_from_selection_owner() needs m_lock mutex locked

Match the number of CoInitialize/CoUninitialize calls

win: Don't create a different header for custom clipboard formats dep…

…ending on platform (32-bit vs 64-bit)

Our custom clipboard formats on Windows has a uint64_t header (it was
size_t before) that indicates how many bytes contains the clipboard
data. We cannot use size_t if we are planning to use a 32-bit version
and 64-bit version of our application at the same time and copy &
paste data between both apps.

Avoid double-unlock of the Manager's mutex after get_data_from_select…

…ion_owner() and then lock::~impl()

This patch fixes the main issue reported in dacap#26

x11: Don't clear selection owner (CLIPBOARD_MANAGER) content on exit