GLES 3.1 requires that the maximum framebuffer width & height be
at least as large as the maximum texture size. Vulkan doesn't have
that requirement and for SwiftShader the values are different
(4096 for FB, 8192 for texture). Use the smaller of the two to
keep things consistent.

Test:
  angle_deqp_gles31_tests --use-angle=swiftshader --gtest_filter=dEQP-GLES31.functional.state_query.integer.max_framebuffer_*

Bug: angleproject:4102
Change-Id: Ie04a5272084c5f6a19ac06d952281b7df9fe9a06
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1917130
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>