Clear attachments in MVKCmdBeginRenderPass between invocations.

Fixes crash due to incorrect number of attachments.
Update What's New document with VK_KHR_imageless_framebuffer extension.

Docs/Whats_New.md

@@ -18,6 +18,8 @@ MoltenVK 1.1.4
 
 Released TBD
 
+- Add support for extensions:
+	- `VK_KHR_imageless_framebuffer`
 - Make `vkGetPastPresentationTimingGOOGLE()` queuing behavior compliant with Vulkan spec.
 - Expose `vkGetIOSurfaceMVK()` and `vkUseIOSurfaceMVK()` without requiring _Objective-C_.
 - Support _Xcode 12.5_ build settings, build warnings, and SDK change to availability of

MoltenVK/MoltenVK/Commands/MVKCmdRenderPass.mm

@@ -58,6 +58,7 @@ VkResult MVKCmdBeginRenderPass<N_CV, N_A>::setContent(MVKCommandBuffer* cmdBuff,
 		_clearValues.push_back(pRenderPassBegin->pClearValues[i]);
 	}
 
+	_attachments.clear();	// Clear for reuse
 	bool imageless = false;
 	for (auto* next = (const VkBaseInStructure*)pRenderPassBegin->pNext; next; next = next->pNext) {
 		switch (next->sType) {