Added tests for OES_EGL_image_external
Added tests for OES_EGL_image_external

Extented EGL image format tests to use external images.

VK-GL-CTS Issue: 2671

Affects:

dEQP-EGL.functional.image.modify.*

Components: EGL
Change-Id: I7e8f717b1214f183e8478c0d4496e637033e229f

KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS