remove allocation from growLTRB; remove dynamism in vector_math

[yjbanov]

Optimizes picture recording by speeding up paint bounds estimation:

• Avoid allocating temporary matrices and vectors on the _PaintBounds hot paths.
• Remove dynamism in vector math.

Here's the A/B comparison on the bench_picture_recording benchmark (see flutter/flutter#54908):

Score	Average A (noise)	Average B (noise)	Speed-up
bench_picture_recording.html.recordPaintCommands.average	3393.82 (3.49%)	1755.48 (3.53%)	1.93x	
bench_picture_recording.html.estimatePaintBounds.average	0.00 (0.00%)	3.01 (15.88%)	0.00x

(i.e. it speeds it up by ~2x)

Update (2020-04-22): the change has propagated to the benchmark and the results confirm that the optimization is effective.

[yjbanov]

Updated the description with benchmark results from the devicelab now that this change propagated to the framework.