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