Remove sha256 from dependencies

[fschutt]

• The shaders are not hashed by their content anymore, only by their file name / their imports - i.e. a shader "a.glsl" that imports "b.glsl" get a unique ID based on the combination ["a.glsl", "b.glsl"] (instead of a SHA hash of the file contents of a.glsl and b.glsl). This also avoids possible collisions.
• The "program digest" is now just a shader ID, there was a check for checking whether the source of the loaded shader is equal to the source of the included shader (which I thought was fairly useless, since those shaders are always the same, so the check can never fail?). So I've removed that check, but I'm not sure if it had historical reasons.

This is a WIP PR, just to give an update, doesn't compile yet, closes #3586.

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This change is 

[bors-servo]

☔ The latest upstream changes (presumably #3727) made this pull request unmergeable. Please resolve the merge conflicts.

[community-tc-integration]

Submitting the task to Taskcluster failed. Details

Taskcluster-GitHub attempted to create a task for this event with the following scopes:

[
  "assume:repo:github.com/servo/webrender:pull-request",
  "queue:route:statuses",
  "queue:scheduler-id:taskcluster-github"
]

The expansion of these scopes is not sufficient to create the task, leading to the following:

Client ID static/taskcluster/github does not have sufficient scopes and is missing the following scopes:

{
  "AllOf": [
    "queue:route:index.garbage.webrender.ci.fschutt.remove-sha2.linux-release",
    {
      "AnyOf": [
        {
          "AnyOf": [
            "queue:create-task:highest:unknown/unknown",
            "queue:create-task:very-high:unknown/unknown",
            "queue:create-task:high:unknown/unknown",
            "queue:create-task:medium:unknown/unknown",
            "queue:create-task:low:unknown/unknown",
            "queue:create-task:very-low:unknown/unknown",
            "queue:create-task:lowest:unknown/unknown"
          ]
        },
        {
          "AnyOf": [
            "queue:create-task:unknown/unknown",
            {
              "AllOf": [
                "queue:define-task:unknown/unknown",
                "queue:task-group-id:taskcluster-github/DTmTffzcRQ-WPq6YqW_-2A",
                "queue:schedule-task:taskcluster-github/DTmTffzcRQ-WPq6YqW_-2A/DTmTffzcRQ-WPq6YqW_-2A"
              ]
            }
          ]
        }
      ]
    }
  ]
}

This request requires the client to satisfy the following scope expression:

{
  "AllOf": [
    "queue:route:index.garbage.webrender.ci.fschutt.remove-sha2.linux-release",
    "queue:route:statuses",
    {
      "AnyOf": [
        {
          "AllOf": [
            "queue:scheduler-id:taskcluster-github",
            {
              "AnyOf": [
                "queue:create-task:highest:unknown/unknown",
                "queue:create-task:very-high:unknown/unknown",
                "queue:create-task:high:unknown/unknown",
                "queue:create-task:medium:unknown/unknown",
                "queue:create-task:low:unknown/unknown",
                "queue:create-task:very-low:unknown/unknown",
                "queue:create-task:lowest:unknown/unknown"
              ]
            }
          ]
        },
        {
          "AnyOf": [
            "queue:create-task:unknown/unknown",
            {
              "AllOf": [
                "queue:define-task:unknown/unknown",
                "queue:task-group-id:taskcluster-github/DTmTffzcRQ-WPq6YqW_-2A",
                "queue:schedule-task:taskcluster-github/DTmTffzcRQ-WPq6YqW_-2A/DTmTffzcRQ-WPq6YqW_-2A"
              ]
            }
          ]
        }
      ]
    }
  ]
}

---

• method: createTask
• errorCode: InsufficientScopes
• statusCode: 403
• time: 2020-02-13T18:39:55.414Z

[fschutt]

PR seems to be borked. I'll create a new one