Flexible buffer bindings

[lshoek]

Removes restriction that would assert that all buffers bound to bindings would have the same size as specified by the shader declaration. Buffer declarations are also permitted to have an element size of zero. This makes it possible to define buffer bindings without specifying the size in the shader:

layout(std430) restrict readonly buffer InPositions
{
	vec4 positions[];
};

We only check that the buffer stride equals the stride in the declaration.

Also:

• Solve some validation errors due to insufficient render target clear colors (these must be greater than or equal to the number of attachments)
• Added a texture copy function that wraps vkCmdCopyImage

[cklosters]

I support the changes, see comments

[lshoek]

Made changes to naprender such that image layouts are properly tracked. NAP always assumed image layout SHADER_READ after any render pass, and would not update nap::ImageData. This can no longer fly as we want to perform other operations on textures such as clear/blit/copy without having to guess the image layout.

I ran into some of there problems on a project where I implemented render target content conservation. Now merging this back into NAP, so nap::RenderTarget now has the property Clear (true by default) that lets you avoid clearing the render target! Very useful in some cases like feedback!