Add support for UNSIGNED_BYTE textures

[nsthorat]

This enables iOS.

We also add new unit test capability to run tests against multiple environment features.

This change is 

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[dsmilkov]

Reviewed 34 of 38 files at r5.
Review status: all files reviewed at latest revision, 6 unresolved discussions.

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src/environment.ts, line 34 at r5 (raw file):

writing

change to 'writing and reading'

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src/environment.ts, line 123 at r5 (raw file):

  const frameBufferComplete =
      (gl.checkFramebufferStatus(gl.FRAMEBUFFER) === gl.FRAMEBUFFER_COMPLETE);

not in this PR, but would be good to see if this catches safari desktop. you might need to call gl.readPixels to test

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src/test_util.ts, line 159 at r5 (raw file):

(testName: string, tests: Tests[], features?: Features)

extract executor signature into a type

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src/math/math_test.ts, line 199 at r5 (raw file):

basicLSTMCell

debug mode here and below

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src/math/pool_test.ts, line 179 at r5 (raw file):

// math.minPool

math.minPool shows up twice, remove this block

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src/math/unaryop_test.ts, line 87 at r5 (raw file):

// math.abs

math.abs repeated.remove block

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Comments from Reviewable

[dsmilkov]

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Review status: all files reviewed at latest revision, 6 unresolved discussions.

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Comments from Reviewable

[nsthorat]

Review status: 29 of 34 files reviewed at latest revision, 6 unresolved discussions.

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src/environment.ts, line 34 at r5 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

writing

change to 'writing and reading'

Done.

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src/environment.ts, line 123 at r5 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

not in this PR, but would be good to see if this catches safari desktop. you might need to call gl.readPixels to test

Added to my todo list.

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src/test_util.ts, line 159 at r5 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

(testName: string, tests: Tests[], features?: Features)

extract executor signature into a type

Done.

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src/math/math_test.ts, line 199 at r5 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

basicLSTMCell

debug mode here and below

Done.

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src/math/pool_test.ts, line 179 at r5 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

// math.minPool

math.minPool shows up twice, remove this block

Done.

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src/math/unaryop_test.ts, line 87 at r5 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

// math.abs

math.abs repeated.remove block

Done.

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Comments from Reviewable