Fix emulated compressed format issues

Fix emulated compressed format issues

When a compressed texture format is emulated, it's possible that the
compress data we get from a glGetCompressedTexImage call is not the
same as what was send through glCompressedTextureSubImage.

So the test needs to be update to handle this situation

KHR-GL45.direct_state_access.textures_get_image now uses
GL_COMPRESSED_RGBA_BPTC_UNORM instead of GL_COMPRESSED_RGB8_ETC2.
GL_COMPRESSED_RGBA_BPTC_UNORM should not be emulated

KHR-GL45.direct_state_access.textures_compressed_subimage store
source and destination compressed texture data.
It also now validate subregion transfert.

Affects:

KHR-GL45.direct_state_access.textures_compressed_subimage
KHR-GL45.direct_state_access.textures_get_image

Components: OpenGL

VK-GL-CTS issue: 491

Change-Id: I0293dd987fc03b719513ea22526f9614d6a9c106