Start work on implementing frame transitions

Also ensure that the correct OS Window is current when uploading images
to GPU.

kitty_tests/graphics.py

@@ -15,8 +15,7 @@ from typing import NamedTuple
 
 from kitty.constants import cache_dir
 from kitty.fast_data_types import (
-    load_png_data, parse_bytes, set_send_to_gpu, shm_unlink, shm_write,
-    xor_data
+    load_png_data, parse_bytes, shm_unlink, shm_write, xor_data
 )
 
 from . import BaseTest
@@ -26,8 +25,6 @@ try:
 except ImportError:
     Image = None
 
-set_send_to_gpu(False)
-
 
 def relpath(name):
     base = os.path.dirname(os.path.abspath(__file__))