Bug fix in unpack buffer validation For an implementation that supports GL_EXT_buffer_storage extension it is valid to read or write from a buffer that is partially or fully mapped if it was allocated by a call to glBufferStorageEXT with the GL_MAP_PERSISTENT_BIT_EXT included in <flags> Bug: angleproject:5056 Bug: angleproject:6689 Test: BufferStorageTestES3.TexImage2DPixelUnpackBufferMappedPersistently Change-Id: Ia4b6967aab02bbfb101d5253b9c83d314bc92f5f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3278482 Commit-Queue: Mohan Maiya <m.maiya@samsung.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Charlie Lao <cclao@google.com>