[osx] Fix memory leaks using -fobjc-arc correctly

The -fobjc-arc flags must be specified for the compiler, not in the
linker. This generates a lot of memory leaks when we copy content into
the clipboard (just testing copy/put_image examples with a for loop to
repeat the action can easily show this leak).

This might be the reason of some memory leaks reported on the macOS
port of Aseprite: aseprite/aseprite#4251

Use target_sources & target_compile_definitions in CMake file

Add hmodule for WinXP support to auto-free the loaded dll + check suc…

…cess of each step

[win] Fix bug pasting 8bpp images

Fixes a bug copying 8bpp images (e.g. from YY-CHR program).
Regression introduced in a834ed5

Related issues:
aseprite/aseprite#4029
https://community.aseprite.org/t/6842

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 #26

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