gfx: Rename WebrenderSurfman to RenderingContext and move to gfx (#31184)

This is a small cleanup that moves and renames this class. The rename is
simply because we are exposing a lot about the details of Servo's
rendering in the API and it makes sense to start thinking about
abstracting that away a bit.

This also moves the struct to `gfx`, which does have an effect on
Servo's dependency graph. This adds a new dependency on gfx to
`compositing`, but `compositing` had a transitive dependency on
gfx before through `canvas`.

Cargo.lock

@@ -640,7 +640,6 @@ dependencies = [
  "time 0.1.45",
  "webrender",
  "webrender_api",
- "webrender_surfman",
  "webrender_traits",
  "webxr",
  "webxr-api",
@@ -858,6 +857,7 @@ dependencies = [
  "embedder_traits",
  "euclid",
  "fnv",
+ "gfx",
  "gfx_traits",
  "gleam",
  "image",
@@ -879,7 +879,6 @@ dependencies = [
  "toml 0.5.9",
  "webrender",
  "webrender_api",
- "webrender_surfman",
  "webxr",
 ]
 
@@ -901,7 +900,6 @@ dependencies = [
  "servo_url",
  "style_traits",
  "webrender_api",
- "webrender_surfman",
 ]
 
 [[package]]
@@ -2004,6 +2002,7 @@ dependencies = [
  "servo_url",
  "smallvec",
  "style",
+ "surfman",
  "truetype",
  "ucd",
  "unicode-bidi",
@@ -3351,7 +3350,6 @@ dependencies = [
  "webgpu",
  "webrender",
  "webrender_api",
- "webrender_surfman",
  "webrender_traits",
  "webxr",
  "webxr-api",
@@ -6858,14 +6856,6 @@ dependencies = [
  "lazy_static",
 ]
 
-[[package]]
-name = "webrender_surfman"
-version = "0.0.1"
-dependencies = [
- "euclid",
- "surfman",
-]
-
 [[package]]
 name = "webrender_traits"
 version = "0.0.1"