[Impeller] Share LazyGlyphAtlas across EntityPass

[luckysmg]

List which issues are fixed by this PR. You must list at least one issue.
flutter/flutter#116815

Now, every EntityPass will create a single LazyGlyphAtlas to use in Canvas::DrawTextFrame, but it seems that there is no need for creating a LazyGlyphAtlas for per EntityPass, and I think maybe we can create a single LazyGlyphAtlas to reuse it across EntityPass.

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[jonahwilliams]

I'm not sure that I understand the change that this PR is trying to make.

LazyGlyphAtlas is attached to the entity pass allowing it to be used across multiple text content canvas operations. Moving it to the canvas makes it harder to reuse.

[luckysmg]

Hi @jonahwilliams
Now, per EntityPass will create a LazyGlyphAtlas. (If there are two EntityPass, will create two LazyGlyphAtlas).
If we move it to canvas, multiple EntityPass can reuse this single LazyGlyphAtlas. LMK if I missed something O(∩_∩)O

[jonahwilliams]

Oh I see, sorry I misunderstood. yes that will accomplish reuse.

We're going to end up with a few more objects that can be shared across entity pass, the tesselator to start with, and at some point some sort of geometry cache too. Is canvas.cc a reasonable place to put these for now, or should we create a separate class to hold them @chinmaygarde ?