If requested then cache unprotected buffers even in protected mode

Protected and normal GLContexts belong to the same share_context which
allows them to share textures between them.  As well as allowing us to
add/remove unprotected textures from either context.

Bug: 190567761
Bug: 187870007
Bug: 190628682
Test: librenderengine_test
Change-Id: I1bada6dea04891149f84ca27080d57322c8686da