Implement save and restore functions on mock canvas so their effects can be tested

[luanpotter]

I'm writing a PR on flame to fix an issue I found where components are not properly rendered in priority order if they are HUD.
Since that involves checking if the camera is applied or not, I am modifying this so that not only the save/restore count as number is saved, but the transformations are actually applied.

This has small implications to all our other existing tests, basically because of two things:

1. we always clear up the camera at the end of the render phase, which requires a new assertion for tests using matchExactly:
   before:

 .. camera is set
 .. your render checks

after:

 .. camera is set
 .. your render checks
 .. camera is cleared

2. since we collapse adjacent transform operations, in some camera tests we were not actually rendering any component, and just checking that the camera transform is applied; however due to the collapse and my optimization on the render flow, the camera was no longer applied needlessly, so all camera tests had to be modified to actually have a component (most already had)

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For the context of how this is used, check flame-engine/flame#1148

[luanpotter]

I just found that its much easier to read like this when there are a handful of commands

[luanpotter]

a public transform conflicts with the method name so I changed to matrix