What exactly does VK_NV_linear_color_attachment do?

The `VK_NV_linear_color_attachment` extension seems to claim it enables using linearly tiled images as color attachment. But that does not seem to have been forbidden before.

If anything the vanilla 1.0 spec says it is already allowed:

> These values **may** be set in `linearTilingFeatures` and `optimalTilingFeatures`, specifying that the features are supported by images or image views created with the queried `vkGetPhysicalDeviceFormatProperties`:
> - [...]
> - VK_FORMAT_FEATURE_COLOR_ATTACHMENT_BIT

Then it adds `VK_FORMAT_FEATURE_2_LINEAR_COLOR_ATTACHMENT_BIT_NV`. I don't see the added value over `VK_FORMAT_FEATURE_COLOR_ATTACHMENT_BIT`.

One would think then at least the extension would guarantee a blanket support for the linear images without a need for `vkGetPhysicalDeviceImageFormatProperties`. But that wording seems unchanged:

> Creation of images with tiling `VK_IMAGE_TILING_LINEAR` **may** not be supported unless other parameters meet all of the constraints:
> - [...]
> - `usage` only includes `VK_IMAGE_USAGE_TRANSFER_SRC_BIT` and/or `VK_IMAGE_USAGE_TRANSFER_DST_BIT`