backend: allow transparent windows to clip other windows

Transparent windows usually blends on top of other windows, this commit
adds an option to make transparent windows clip other windows like
non-transparent windows do.

Closes yshui#265

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

Changes

Only change since v7.4 is the rename from `compton` to `picom`, which is
backported from the `next` branch.

As a user, you don't need to update to this version.

As a distro packager, you should only package this version if you want
to adopt "picom" as the package name.

v7.4

- Fix several memory leaks, thanks to @liskin (yshui#236)

v7.3

- Build changes:
  - Vendor test.h so people building from the release tarballs doesn't
    have to clone that repository with git

v7.2:

- Fixes:
  - Stopped compton from sometimes freezing the X server when dbus is
    enabled.
  - Fixed the example dbus scripts.

v7.1

- Fixes:
  - Fixed yshui#209
  - Fixed assertion failures, bugs, and memory leaks in experimental
    backends

gl_common: use texelFetch

This relieves us from the need to normalize texture coordinates.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

new backend: ignore window tranparency info from the backend

Doing this forces compton to blur every windows' background.

However, doing this can be beneficial, we will figure out a way
to do this properly in the future.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

Stop debug mode from being used without the new backends

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>