New interfaces added and stub methods implemented for OffscreenCanvas API

OffscreenCanvas implementaion - 25 bugs removed towards a successful build.

OffscreenCanvas API Implemented with stub methods, build successful

Conflicts removed from master, OffscreenCanvas changes ready for merge

Revert "Conflicts removed from master, OffscreenCanvas changes ready for merge"
Will rebase branch and resolve conflicts.
This reverts commit 7357f2e.

Changes reverted in conflicting files. OffscreenCanvas API ready to be merged.