[canvaskit] Remove the need for users to keep track of contexts.

We'll switch to the correct context when necessary (e.g. before
calls that talk to the GPU). This is achieved by adding in
calls at the JS layer to switch the context before making a call
that is known to talk to the GPU (e.g. draw calls on SkCanvas).

Another implementation that was considered was to add a C++
shim in GrGLInterface that would switch the context before
every call in the GPU - however, that seemed too difficult
and would add extra overhead if a single draw* call talks
to the GPU multiple times.

Bug: skia:12255
Change-Id: I96e4c6b41a5bfcc9913aeaca7ccb125358048ad3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/432136
Reviewed-by: Brian Salomon <bsalomon@google.com>
12 files changed