Implement capturing GLES1 specific state by checking the difference to
the default state using getEnableFeature.

When getEnableFeature is called from a context version higher than 1 and
a GLES1 state is requested, an assertion is hit. Therefore the context
version needs to be determined before calling getEnableFeature.

Use the available isTextureTargetEnabled function to determine
GL_TEXTURE_2D and GL_TEXTURE_CUBE_MAP states in getEnableFeature.

Return false in isTextureTargetEnabled when mTextUnitEnables is empty,
so the vector is not accessed when not initialized, which was the case
for a default state.

Bug: angleproject:5893
Change-Id: I66ee41c3bf7a8e1f04a8a4ce0461fddc16f9a013
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2877237Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Lubosz Sarnecki <lubosz.sarnecki@collabora.com>