wayland/screencopy: use linear texture filtering over nearest

Fixes pixelated views at scaled resolutions.

changelog/next.md

@@ -64,6 +64,7 @@ set shell id.
 - Fixed partial socket reads in greetd and hyprland on slow machines.
 - Worked around Qt bug causing crashes when plugging and unplugging monitors.
 - Fixed HyprlandFocusGrab crashing if windows were destroyed after being passed to it.
+- Fixed ScreencopyView pixelation when scaled.
 
 ## Packaging Changes