Fixed synchronization issue on MacOS

Fixed synchronization issue on MacOS

Because of the way SwiftShader is integrated into Chromium on MacOS,
a synchronization issue exists when Chromium attempts to use an
IOSurface before SwiftShader is done rendering into it. In order to
solve this, all draw calls that end up rendering into an IOSurface
must be synchronized within SwiftShader and can't yield to Chromium
until rendering is complete.

Bug chromium:847094

Change-Id: If2dba4fa998e7437ec414d3b4aff9e8b19ecc128
Reviewed-on: https://swiftshader-review.googlesource.com/19188
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>