Reduce read pixel calls in copy image tests
Reduce read pixel calls in copy image tests

Reduce glReadPixels calls in the copy image tests by rendering multiple
texture sub-images for each read back. Significant improvement for
certain platforms, no impact on others.

Bug: 33965234
Test: Run tests on Pixel C and Pixel XL, Check that old N9 results still
have same failures. Spot checks with 240x200 target surface.
Change-Id: I133c56b7855a664f164f1eb0b1307259462e098f

KhronosGroup/Vulkan-CTS,chadversary/deqp,KhronosGroup/Vulkan-CTS,chadversary/deqp,chadversary/deqp,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,chadversary/deqp,chadversary/deqp,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,KhronosGroup/Vulkan-CTS,chadversary/deqp,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,googlestadia/VK-GL-CTS,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS