GLES1 support in multi-context D3D mode is broken prior to this
change as state in the GLES1Renderer is not being properly
dirtied on context switch. The immediate result is a crash on
draw commands due to a null program.

Introduce a new dirty bit to specify that the program is dirty
and should be re-initialized.

Test: GLES1 applications work with the D3D11 backend in multi-context
mode.

Bug: b/191158936
Change-Id: Ib53c1471514ff2cf36ef9fc0d6bfb99a8bb7cf66
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2969789Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Doug Horn <doughorn@google.com>