x/exp/shiny/driver/gldriver: fails to build on darwin/arm64 because its gl dependency doesn't support darwin/arm64 yet

[dmitshur]

2020-12-15T15:35:30-b5a6e24/darwin-arm64-11_0-toothrot

# golang.org/x/mobile/gl
In file included from /tmp/buildlet/gopath/pkg/mod/golang.org/x/mobile@v0.0.0-20190719004257-d2bd2a29d028/gl/work.go:29:
./work.h:18:10: fatal error: 'OpenGLES/ES2/glext.h' file not found
#include <OpenGLES/ES2/glext.h>
         ^~~~~~~~~~~~~~~~~~~~~~
1 error generated.

Similar to #43221, but for OpenGL.

Support for darwin/arm64 needs to be added to golang.org/x/mobile/gl, then x/exp needs to pull in the new version.

[gopherbot]

Change https://golang.org/cl/278779 mentions this issue: gl: add darwin/arm64 (macOS) support

[gopherbot]

Change https://golang.org/cl/278761 mentions this issue: shiny/driver/gldriver: add darwin/arm64 (macOS) support