Dynamic rectangle count for VK_EXT_discard_rectangles

Right now clients wishing to use `VK_EXT_discard_rectangles` need to provide a static `discardRectangleCount` at pipeline creation stage. This is an issue if the number of rectangles is not known in advance.

Would it be reasonable to add a new `vkCmdSetDiscardRectangleCountEXT` command to dynamically set the number of discard rectangles?

Alternatively, as a workaround, can "no-op" rectangles with a zero width/height be provided?

Start by reviewing the VK_EXT_discard_rectangles specification and the issue's questions about pipeline creation and zero-size rectangles. A complete outcome would clarify whether a dynamic discard-rectangle count is appropriate and document the accepted behavior or workaround.

Written by the indexing model from the issue text.