Switch to the SourceSDK for HL1

[SaintWish]

It'd be useful for us in the future if we switched to this SDK instead of the current HL1 SDK https://github.com/tmp64/hl1_source_sdk

[tschumann]

This SDK is an addition not a replacement - it adds all the plumbing required to load VGUI2, so effectively it blocks #181

FWGS/hlsdk-portable#447 shows how to add it to client.dll

[tschumann]

Started on this with #287 but it looks like some stuff is missing from the latest Half-Life SDK to allow this.

https://hlcoders.valvesoftware.narkive.com/cPZseNbN/goldsrc-vgui2-headers-libs-screen-overlays has a bit more information too.