add texture array support to vulkan readback

[pgruenbacher]

add in numLayers to ReadBackVK utility class so that it properly copies texture arrays when called by bgfx::readTexture.

tested in examples-20-picking to still work

The ReadBackVK class could probably be made to be more elegant as well as to handle texturecube but I don't have a good test-case for that.

[bkaradzic]

cc @pezcode

[pezcode]

LGTM, modulo the one suggested change 👍

[pezcode]

The first 1 (_levelCount) should still be there, to only add a layout transition for _mip. Otherwise the driver will transition all higher mip levels as well.

Same up top.

[pgruenbacher]

yep agreed. i changed it back to level=1, baseArrayLayer=1, layerCount = VK_REMAINING_ARRAY_LAYERS for imageMemoryBarrier

[bkaradzic]

Make sure this have the same behavior on other backends too.

[pgruenbacher]

Make sure this have the same behavior on other backends too.

it should for opengl

looking at metal it seems to be the same behavior
more importantly since readTexture only accepts mip as an argument, the expected behavior should be to copy all of the data and not just a single slice of the array. otherwise no other way to get the data

[bkaradzic]

more importantly since readTexture only accepts mip as an argument

I think maybe API should change... Intention was always to read single 2D slice, not complete chain. Something like:

	uint32_t readTexture(
		  TextureHandle _handle
		, void* _data
		, uint16_t _layer
		, uint8_t _mip
		);