Test that buffer writes are preserved after another range is mapped. (#610)
Test that buffer writes are preserved after another range is mapped. (#610)

Completes a TODO in api,operation,buffers,map:*

Uses mappedAtCreation or mapAsync to write to various ranges of variously-sized buffers, then uses mapAsync to map a different range and zero it out. Finally uses expectGPUBufferValuesEqual to verify that contents originally written outside
the second mapped range were not altered.
gpuweb/cts,gpuweb/cts,gpuweb/cts