GUI: Inconsistent default scaler behaviour

[sev-]

ScummVM behaves differently when a highres game is launched from the launcher or from the command line.

Prerequisites:
640x480 game
graphics mode: <default>
scaler: normal, 2x
game gfx: do not override

Steps to reproduce:

1. Launch the game from the launcher -> 640x480 window
2. Launch the game from the command line -> 1280x960 window

Additional side effect after step 2: game gets "override gfx" ticked and normal2x set as the scaler.

Possible solutions:

1. Disable and remove scaler when <default> gfx mode is selected
2. Force scaler to <default> when <default> gfx mode is selected
3. Ignore scaler settings when <default> gfx mode is selected just as we do now when launched from scaler