If an App repeatedly issues GL commands like glTextImage2D without a
finish/flush/draw, it's possible for ANGLE to exhaust the available
Vulkan memory allocations and exceed
VkPhysicalDeviceLimits::maxMemoryAllocationCount. When this occurs,
the Vulkan validation layers will trigger an error and cause dEQP
tests to fail.

This change will query the backend if a flush() should be performed
during each of the GL delete calls, and perform it if necessary. This
will cause a queue submission and a Serial increment, allowing the
allocated memory to be freed, preventing the validation errors.

Bug: angleproject:3818
Test: KHR-GLES3.copy_tex_image_conversions.forbidden.renderbuffer_cubemap_*
Change-Id: I26d0a47aa7bca10c25bc8141f1523afbab0b3b5b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1834781
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>