```Add atan_ primitive to SkRasterPipeline.

The atan approximation logic was lifted from SkVM.

Change-Id: Id2dcc369a84666d9dafed6d7365988f906e89b18
```
```diff --git a/src/opts/SkRasterPipeline_opts.h b/src/opts/SkRasterPipeline_opts.h
index cf6a085..0ec9146 100644
--- a/src/opts/SkRasterPipeline_opts.h
+++ b/src/opts/SkRasterPipeline_opts.h
```
```@@ -1502,6 +1502,34 @@
return x;
}

+/*  Use 4th order polynomial approximation from https://arachnoid.com/polysolve/
+        with 129 values of x,atan(x) for x:[0...1]
+    This only works for 0 <= x <= 1
+ */
+SI F approx_atan_unit(F x) {
+    // y =   0.14130025741326729 x⁴
+    //     - 0.34312835980675116 x³
+    //     - 0.016172900528248768 x²
+    //     + 1.00376969762003850 x
+    //     - 0.00014758242182738969
+    return x * (x * (x * (x * 0.14130025741326729f - 0.34312835980675116f)
+                                                   - 0.016172900528248768f)
+                                                   + 1.0037696976200385f)
+                                                   - 0.00014758242182738969f;
+}
+
+// Use identity atan(x) = pi/2 - atan(1/x) for x > 1
+SI F atan_(F x) {
+    I32 neg = (x < 0.0f);
+    x = if_then_else(neg, -x, x);
+    I32 flip = (x > 1.0f);
+    x = if_then_else(flip, 1/x, x);
+    x = approx_atan_unit(x);
+    x = if_then_else(flip, SK_ScalarPI/2 - x, x);
+    x = if_then_else(neg, -x, x);
+    return x;
+}
+
// Used by gather_ stages to calculate the base pointer and a vector of indices to load.
template <typename T>
SI U32 ix_and_ptr(T** ptr, const SkRasterPipeline_GatherCtx* ctx, F x, F y) {
```
```diff --git a/tests/SkRasterPipelineOptsTest.cpp b/tests/SkRasterPipelineOptsTest.cpp
index 926a825..438ee6b 100644
--- a/tests/SkRasterPipelineOptsTest.cpp
+++ b/tests/SkRasterPipelineOptsTest.cpp
```
```@@ -121,3 +121,16 @@
}
}
}
+
+DEF_TEST(SkRasterPipelineOpts_Atan, r) {
+    using F = SK_OPTS_NS::F;
+
+    constexpr float kTolerance = 0.00175f;
+    for (float x = -10.0f; x <= 10.0f; x += 0.1f) {
+        F result = SK_OPTS_NS::atan_(x);
+        F expected = atanf(x);
+        F delta = SK_OPTS_NS::abs_(expected - result);
+
+        REPORTER_ASSERT(r, SK_OPTS_NS::all(delta < kTolerance));
+    }
+}
```