Cells don't render in second window if both windows contain graphics and window is semi-transparent

[ctrlcctrlv]

Config options different from defaults:
background_opacity    0.5
cursor_blink_interval 0.0
enable_audio_bell     False
font_family           Nimbus Mono PS
font_size             24.0
update_check_interval 24.0
visual_bell_duration  0.2

Arch Linux, 1b5daf9

Discovered while working on #163, complicates that issue as the affected code path always runs with my WIP patch.

Screenshot:

I think this happens because there's only one offscreen frame buffer (see shaders.c:157), but each window should have its own offscreen framebuffer. Maybe? Could be totally wrong about the cause.

Reproduction steps:

1. With settings above, most important being background_opacity:
2. Run kitty +icat /path/to/image.png
3. Open new window in same kitty process (Ctrl+Shift+n)
4. Run kitty +icat /path/to/image.png
5. The cells should stop rendering in the second window as my screenshot shows

[ctrlcctrlv]

I was right about the cause and am preparing a PR as we speak.