Add support for viewport scaling to exported HTML

[pushfoo]

Expected behavior

Near full-width scaling while in portrait orientation or full-height scaling when in landscape orientation, optionally with a tiny bit of margin left over, regardless of whether the file is used bare or as an embed for itch.io.

Actual behavior

I know the issue affects Android Chrome and Chrome's desktop mobile emulation mode. I don't have access to a recent iOS device to verify the issue on those. The game still appears to be executing rather than frozen, even if parts are cut off.

If it's troublesome to come up with a single style that works for both itch.io embed and a bare file, then a practical solution could be to make a tailored itch.io export option in the web IDE.

Itch.io uploads

Portrait Orientation

The screen's and keypad's sides are cut off when the screen is narrower than 516px. I am unsure whether this corresponds to actual pixels the units in the inspector.

Before tapping/clicking to enable the keypad:

After tapping/clicking to enable the keypad:

Landscape orientation

This seems to trigger when the screen is shorter than 480px. I am unsure whether this corresponds to actual pixels the units in the inspector.

Before tapping/clicking to enable the keypad:

After tapping/clicking to enable the keypad:

From local files

Portrait Orientation

Before tapping/clicking to enable the touchpad:

After tapping/clicking to enable the touchpad:

Landscape Orientation

This works completely fine.

[pushfoo]

This is still relevant in 2023, but the nature of the problem has changed since itch's embed handling appears to have changed.

Example:

Vertical view	Horizontal view

I think there might be two steps to this:

1. Add documentation on how to set up an itch.io page with correct browser viewport sizes (Add documentation on itch.io export issues #179)
2. See if there are ways of dealing itch's weird viewport behavior