[test] Add external texture GPU tests

These tests cover the external texture binding resource. They ensure
the WGSL functions `textureDimensions()`, `textureLoad()`, and
`textureSampleBaseClampToEdge()` work as expected for both
`TextureView`s and `ExternalTexture`s bound to external texture
resource bindings.

For external textures, they ensure multiplanar YUV formats work as
expected including handling color space transformation. And that the
provided sample and load transforms correctly handle cropping,
flipping, and rotation.

Author: jamienicol

tests/tests/wgpu-gpu/external_texture/dimensions.wgsl

@@ -0,0 +1,10 @@
+@group(0) @binding(0)
+var tex: texture_external;
+
+@group(0) @binding(1)
+var<storage, read_write> output: vec2<u32>;
+
+@compute @workgroup_size(1)
+fn main(@builtin(global_invocation_id) global_id: vec3<u32>) {
+    output = textureDimensions(tex);
+}

tests/tests/wgpu-gpu/external_texture/load.wgsl

@@ -0,0 +1,12 @@
+@group(0) @binding(0)
+var tex: texture_external;
+
+@group(0) @binding(1)
+var<storage, read> coords: array<vec2<u32>>;
+@group(0) @binding(2)
+var<storage, read_write> output: array<vec4<f32>>;
+
+@compute @workgroup_size(1)
+fn main(@builtin(global_invocation_id) id: vec3<u32>) {
+    output[id.x] = textureLoad(tex, coords[id.x]);
+}