<!-- Please describe your changes on the following line: -->
Currently `bound_texture_2d` and `bound_texture_cube_map` fields are used to restore texture states and to get the current bindings in GetParams(...). But as soon as active texture is changed all the values can become dirty, leading to broken textures in some demos or invalid getParam(...) call results.
This PR implements the texture binding association with the texture units.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).
<!-- Either: -->
- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____
<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
Source-Repo: https://github.com/servo/servo
Source-Revision: ffa03380caee4cf1ffd0c43a219936cb9a2e0b85
--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : ec3106ad7b93adf69aaedf9007d3af8bcac2d6b0
735f73f9e3