Add support for fences in WebGL 2.0. Turn off query timers.

[nsthorat]

• Adds fenceSync + getBufferData in WebGL 2.0 when downloading data from a texture.
• Abstracts fences to either use fenceSync (gl 2.0) or EXT_disjoint_query_timer (gl 1.0) depending on the device. Fences always go through the binary search queryer. Timers now always are polled independently (since the GPU time is not a function of the CPU time).
• Turns off WEBGL_DISJOINT_QUERY_TIMER_EXTENSION_VERSION for safety. We've made significant changes to the code paths under this path but have had no way of testing it. Once Chrome reenables EXT_disjoint_query_timer (Re-enable WEBGL_DISJOINT_QUERY_TIMER_EXTENSION_VERSION when chrome enables EXT_disjoint_query_timer tfjs#544) we'll reenable the bit.
• Throw an error for GL 2.0 + no floats. This currently will not work. If we ever need this, we'll come back to it.
• Adds typings for WebGL 2.0 and webgl-ext. Verified the compile time does not change.
• Fixes a bug in the non-float download enable code where it creates a tensor with a preexisting DataId, causing a memory leak and an error.

Side effects:

• MathBackendWebGL.readSync() always calls gl.readPixels to a CPU ArrayBuffer. read() calls gl.readPixels into a CPU ArrayBuffer in WebGL 1.0, and calls gl.readPixels into a GPU Buffer and then calls gl.getBufferSubData into the CPU ArrayBuffer. These deviate ever so slightly, but it should not be a big deal (since performance is not critical for readSync())

This change is 

[nsthorat]

Reviewable status: 0 of 1 approvals obtained

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package.json, line 22 at r1 (raw file):

    "@types/seedrandom": "~2.4.27",
    "@types/webgl2": "~0.0.4",
    "@types/webgl-ext": "0.0.29",

this typing is not in google3 yet, I'm working on syncing it.

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src/kernels/webgl/webgl_types.ts, line 18 at r1 (raw file):

 */

export interface WebGLQuery {}

these are now taken care of by the official typings.

[dsmilkov]

Reviewed 9 of 9 files at r1.
Reviewable status: 0 of 1 approvals obtained (waiting on @nsthorat and @dsmilkov)

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src/environment_test.ts, line 59 at r1 (raw file):

    ENV.setFeatures(features);
    // expect to be 1 when https://github.com/tensorflow/tfjs/issues/544 is

prepend a TODO

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src/environment_test.ts, line 86 at r1 (raw file):

    ENV.setFeatures(features);
    // expect to be 2 when https://github.com/tensorflow/tfjs/issues/544 is

same here

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src/kernels/backend_webgl.ts, line 301 at r1 (raw file):

      texShape: [number, number], shape: number[]): Float32Array {
    if (!ENV.get('WEBGL_DOWNLOAD_FLOAT_ENABLED')) {
      const tmpTarget = Tensor.make(shape, {});

maybe flip the if with more lines after the if

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src/kernels/webgl/gpgpu_context.ts, line 247 at r1 (raw file):

          query, ENV.get('WEBGL_DISJOINT_QUERY_TIMER_EXTENSION_VERSION'));
    } else {
      // If we have no way to fence, return true immediately.

extend the comment a bit saying under which condition this happens

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src/kernels/webgl/gpgpu_util.ts, line 309 at r1 (raw file):

            gl2.PIXEL_PACK_BUFFER, bufferSizeBytes, gl.STATIC_DRAW));

    // Copy the texture into the buffer.

made say enqueue to the gpu command buffer, since it's not blocking or immediate