Adds consistent mapping between event received touch id and UI internal touch id, to ensure that first touch id always acts as mouse left.
Updated the UI Drag sample descriptions.
Moved the SDL touch ID -> Urho touch ID logic from UI to Input, so that querying Input/TouchState should give you a touch index that will always be consistent (between TouchDown and TouchUp events) and start from 1.
Merge remote-tracking branch 'hdunderscore/UIDrag_Bugfix'
Fix consistency in touch hover touch ID. Fix returning touch IDs as available. When resetting touches, reset the touch ID mapping also. In UIDrag example, show the actual touch ID's instead of indices, to verify that the ID's stay consistent. Fix UIDrag Lua example erroneously modifying the TouchState's position.
Merge branch 'master' of https://github.com/urho3d/Urho3D
For Travis CI - switch CI build to use Raspberry-Pi build environment.