This constant was set with some wrong assumptions:
1. On Android, recovering was needed.
2. On iOS, OpenGL ES was used when
a. The architecture was 386 or amd64 == an emulator is used
b. The build tag ebitengl was not specified
c. gomobile-build was used
3. On browsers, recovering was needed.
1., 2b, and 2c are correct.
2a. is not correct: Now emulators are available on all the
architectures with both Metal and OpenGL.
3. is not correct: Ebiten no longer recovers the contest lost.
Now, Ebiten can detect a context lost explicitly when
1. On Android
2. On iOS and on gomobile-build
(When gomobile-build is used, OpenGL should always be used)
Based on this fact, this change changes the constant to a variable,
and fixes the logic to set the variable.
151dfffad6