[Impeller] Vulkan external texture composition can only handle shaders with a single sampler.

[chinmaygarde]

This is a TODO introduced in flutter/engine#50730.

Today, when sampling from an external image in a shader in the Vulkan backend, only one sampler may be specified. This is because sampling from an external texture requires the sampler to be immutable and the pipeline to be rebuilt. This pipeline rebuilding mechanism only handles the case of single sampler today.

We have no cases where external texture sampling shaders use more than one sampler. But, if/when we do, we should expect to see errors when sampling from a combined image sampler that expects to be bound in the renderpass.

[chinmaygarde]

Still super low priority but wanted to add a use-case/test-case for this: Cross-fading between two video players in a single draw call.