Fix Image views

[heyx3]

I made some big mistakes when setting these up.

Access Mode

First, the access mode (read, write, or read_write) is not a property of the view itself, but of the view's activation. When activating a view, we need to specify its access mode. When constructing a view, we do not specify an access mode.

On the surface, that's a simple fix: remove "access mode" from the View constructor's parameters, and add it to view_activate()'s parameters. However, in practice there are some difficult architectural and OpenGL questions. Such as:

• If a view is activated for "read" and then elsewhere for "write", or vice-versa, how is that handled? Should we deactivate and then reactivate as "read_write"? What about when one of them is deactivated again?
• Could we get away with always activating as "read_write" and not thinking about the problem any harder?

Format

Meanwhile, something else is missing from image views on construction: a "format". This is how the image appears to the shader, and the only requirement is that the byte size of the apparent pixel format matches the byte size of the true pixel format. For example, you could have an RG16_UINT texture show up as R32_FLOAT in the shader and it work as you'd expect (though different GPU's may have different endianness).

[heyx3]

The question of what happens if you make an Image View handle resident twice with different access modes does not seem to be answered anywhere on the internet. So I'm going to simply forbid activating a texture twice.

I'm also going to provide a default access mode of read_write if one isn't given. This way of doing things is convenient, and not a big deal if it's too permissive (again, as far as I can tell...).