gpu: [compute] add CPU fallback

This change adds a CPU fallback for devices that don't support the old
renderer nor have GPU support for compute programs.

Most of the hard work is implemented in the gioui.org/cpu module. It
uses the SwiftShader project with light modification to output
statically compiled CPU .o files for each compute program.

The CPU fallback only covers Linux and Android on arm, arm64, amd64
architectures. There is no fundamental reason support can't be extended
to other platforms:

- macOS and iOS are probably easy, but it's likely that virtually every
  device has GPU support for compute shaders.
- Windows needs a Cgo-less port, or a build constraint to require a C
  compiler (Gio core doesn't).
- FreeBSD and OpenBSD are probably also easy to do because they're so
  similar to Linux.
- The 386 binaries didn't work properly in my tests, so fixes to
  SwiftShader is probably needed. However, I expect virtually every
  Intel device can run amd64 binaries.

Updates gio#49
Fixes gio#228

Signed-off-by: Elias Naur <mail@eliasnaur.com>

go.sum

@@ -1,6 +1,8 @@
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 dmitri.shuralyov.com/gpu/mtl v0.0.0-20201218220906-28db891af037/go.mod h1:H6x//7gZCb22OMCxBHrMx7a5I7Hp++hsVxbQ4BYO7hU=
+gioui.org/cpu v0.0.0-20210727122813-41509bcd3462 h1:JZyB+d8tPExZHNZwMiGKeeAVd0mkFTc3Zsmegdn178M=
+gioui.org/cpu v0.0.0-20210727122813-41509bcd3462/go.mod h1:DkhBDuHokSMOUxX5LZQ7IcxyJJzs3OON8Z5ojaXUXxo=
 github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
 github.com/BurntSushi/xgb v0.0.0-20160522181843-27f122750802/go.mod h1:IVnqGOEym/WlBOVXweHU+Q+/VP0lqqI8lqeDx9IjBqo=
 github.com/Knetic/govaluate v3.0.1-0.20171022003610-9aa49832a739+incompatible/go.mod h1:r7JcOSlj0wfOMncg0iLm8Leh48TZaKVeNIfJntJ2wa0=