Support swappable textures as the backing for the draw buffer.

[jdm]

This is required to fix servo/servo#21841. When a frame is complete, a different texture is used as the backing for the framebuffer, allowing the complete texture to be read from at WebRender's leisure.

[jdm]

---- tests::test_multithread_sharing_gl2 stdout ----
thread 'tests::test_multithread_sharing_gl2' panicked at 'assertion failed: `(left == right)`
  left: `[0, 0, 0, 0]`,
 right: `[255, 0, 0, 255]`', src/tests.rs:35:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
---- tests::test_multithread_sharing_gl3 stdout ----
thread 'tests::test_multithread_sharing_gl3' panicked at 'assertion failed: `(left == right)`
  left: `[0, 0, 0, 0]`,
 right: `[255, 0, 0, 255]`', src/tests.rs:35:9
---- tests::test_sharing_gl2 stdout ----
thread 'tests::test_sharing_gl2' panicked at 'assertion failed: `(left == right)`
  left: `[128, 0, 0, 63]`,
 right: `[255, 0, 0, 255]`', src/tests.rs:35:9
---- tests::test_sharing_gl3 stdout ----
thread 'tests::test_sharing_gl3' panicked at 'assertion failed: `(left == right)`
  left: `[0, 0, 0, 255]`,
 right: `[255, 0, 0, 255]`', src/tests.rs:35:9
failures:
    tests::test_multithread_sharing_gl2
    tests::test_multithread_sharing_gl3
    tests::test_sharing_gl2
    tests::test_sharing_gl3

[emilio]

Can you make this ColorAttachment::Texture { active: GLuint, complete: GLuint }?

[emilio]

This looks like a separate fix / change, is it needed? If so, a comment in the commit message or the code would be appreciated.