shiny/driver/x11driver: fallback to regular pixmaps if SHM is unavaiable

This PR implement the **TODO** in buffer.go:
```
// TODO: detect if the X11 server or connection cannot support SHM pixmaps,
// and fall back to regular pixmaps.
```
It fixes the issues:
[golang/go#15100](https://github.com/golang/go/issues/15100)
[aarzilli/gdlv#26](https://github.com/aarzilli/gdlv/issues/26)
[aarzilli/gdlv#5](https://github.com/aarzilli/gdlv/issues/5)
[oakmound/shiny#8](https://github.com/oakmound/shiny/issues/8)

@sbinet
@aarzilli
@200sc

Change-Id: I7f157c95928ca5fde015935ea5fe1d1f8b03ea43
GitHub-Last-Rev: 0a5d514a6c5bc0ca7578a7339b99d1e794642d03
GitHub-Pull-Request: golang/exp#9
Reviewed-on: https://go-review.googlesource.com/c/exp/+/213199
Reviewed-by: Nigel Tao <nigeltao@golang.org>
6 files changed
tree: 22daab1a799863d28a42ca3e5c31eca47b5699f1
  1. apidiff/
  2. cmd/
  3. ebnf/
  4. ebnflint/
  5. errors/
  6. fsnotify/
  7. inotify/
  8. io/
  9. mmap/
  10. rand/
  11. shiny/
  12. shootout/
  13. sumdb/
  14. utf8string/
  15. winfsnotify/
  16. .gitattributes
  17. .gitignore
  18. AUTHORS
  19. codereview.cfg
  20. CONTRIBUTING.md
  21. CONTRIBUTORS
  22. go.mod
  23. go.sum
  24. LICENSE
  25. PATENTS
  26. README.md
README.md

exp

This subrepository holds experimental and deprecated (in the old directory) packages.

The idea for this subrepository originated as the pkg/exp directory of the main repository, but its presence there made it unavailable to users of the binary downloads of the Go installation. The subrepository has therefore been created to make it possible to go get these packages.

Warning: Packages here are experimental and unreliable. Some may one day be promoted to the main repository or other subrepository, or they may be modified arbitrarily or even disappear altogether.

In short, code in this subrepository is not subject to the Go 1 compatibility promise. (No subrepo is, but the promise is even more likely to be violated by go.exp than the others.)

Caveat emptor.