Cannot select and scroll beyond the bounds of the viewport in integrated terminal

[weinand]

for #6654:

• VSCode Version: 1.1.0 alpha
• OS Version: Windows 10

to reproduce:

• launch integrated terminal
• run several 'dir' commands to fill the terminal with more lines than visible
• select the lines with the mouse and drag the mouse outside to the viewport to initiate autoscrolling.

Observe: there is no autoscrolling but some other text gets selected.

[Tyriar]

This one is related to limitations of xterm.js

[Tyriar]

Upstream issue xtermjs/xterm.js#69