[Impeller] Vulkan: Re-enable clips pipelines with no color attachments.

[bdero]

We needed to disable this for Vulkan due to validation errors found while running Flutter Gallery: flutter/engine#43905

E/flutter (15691): -----------------------------------------------------------------
E/flutter (15691): [ERROR:flutter/impeller/base/validation.cc(49)] Break on 'ImpellerValidationBreak' to inspect point of failure: 
E/flutter (15691): --- Vulkan Debug Report  ----------------------------------------
E/flutter (15691): |                Severity: Error
E/flutter (15691): |                    Type: { Validation }
E/flutter (15691): |                 ID Name: VUID-vkCmdDraw-renderPass-02684
E/flutter (15691): |               ID Number: 1349015333
E/flutter (15691): |       Queue Breadcrumbs: [NONE]
E/flutter (15691): |  CMD Buffer Breadcrumbs: [NONE]
E/flutter (15691): |         Related Objects: RenderPass [3352410953092073] [EntityPass Render Pass: Depth=0 Count=0], RenderPass [329853488333100] [UNNAMED]
E/flutter (15691): |                 Trigger: Validation Error: [ VUID-vkCmdDraw-renderPass-02684 ] Object 0: handle = 0xbe90000000be9, name = EntityPass Render Pass: Depth=0 Count=0, type = VK_OBJECT_TYPE_RENDER_PASS; Object 1: handle = 0x12c000000012c, type = VK_OBJECT_TYPE_RENDER_PASS; | MessageID = 0x50685725 | vkCmdDraw: RenderPasses incompatible between active render pass w/ VkRenderPass 0xbe90000000be9[EntityPass Render Pass: Depth=0 Count=0] and pipeline state object w/ VkRenderPass 0x12c000000012c[] Attachment 0 is not compatible with 4294967295: The second is unused while the first is not.. The Vulkan spec states: The current render pass must be compatible with the renderPass member of the VkGraphicsPipelineCreateInfo structure specified when creating the VkPipeline bound to VK_PIPELINE_BIND_POINT_GRAPHICS (https://www.khronos.org/registry/vulkan/specs/1.3-extensions/html/vkspec.html#VUID-vkCmdDraw-renderPass-02684)
E/flutter (15691): -------------------------------------------------------

• Fix the underlying problem and re-enable.
• Figure out why the Vulkan goldens did not catch and/or error out when encountering this validation issue.

[flutter-triage-bot]

This issue is marked P1 but has had no recent status updates.

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[jonahwilliams]

@bdero is any part of this issue still relevant, or have we entirely worked around it?

[bdero]

This is still relevant for the stencil phase of StC. But I suspect this would be a huge pain and risk making things a lot worse on some devices for a tenuous improvement on others.

Like we'd probably need to notify the HAL in advance and insert Vulkan subpasses based on changes to attachments in the stream of pipelines used against the RenderPass. But IIUC Vulkan is underspecified here and there's no way to query for whether the driver will fallback to catastrophic deoptimized nightmare behavior if we were to do this.

So unless we have easy alternatives to inserting subpasses in Vulkan 1.1 here, maybe we should just forget about it.