[Impeller] work around for crashy Nexus 5 Driver. (#164040)

Rewrites exp to work around crashes on Nexus 5.

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -41654,6 +41654,7 @@ ORIGIN: ../../../flutter/impeller/compiler/shader_lib/impeller/dithering.glsl +
 ORIGIN: ../../../flutter/impeller/compiler/shader_lib/impeller/external_texture_oes.glsl + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/impeller/compiler/shader_lib/impeller/gaussian.glsl + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/impeller/compiler/shader_lib/impeller/gradient.glsl + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/impeller/compiler/shader_lib/impeller/math.glsl + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/impeller/compiler/shader_lib/impeller/path.glsl + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/impeller/compiler/shader_lib/impeller/prefix_sum.glsl + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/impeller/compiler/shader_lib/impeller/texture.glsl + ../../../flutter/LICENSE
@@ -44568,6 +44569,7 @@ FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/dithering.glsl
 FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/external_texture_oes.glsl
 FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/gaussian.glsl
 FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/gradient.glsl
+FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/math.glsl
 FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/path.glsl
 FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/prefix_sum.glsl
 FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/texture.glsl

engine/src/flutter/impeller/compiler/shader_lib/impeller/BUILD.gn

@@ -13,6 +13,7 @@ copy("impeller") {
     "external_texture_oes.glsl",
     "gaussian.glsl",
     "gradient.glsl",
+    "math.glsl",
     "path.glsl",
     "texture.glsl",
     "tile_mode.glsl",

engine/src/flutter/impeller/compiler/shader_lib/impeller/math.glsl

@@ -0,0 +1,16 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MATH_GLSL_
+#define MATH_GLSL_
+
+// pow(x, y) crashes the shader compiler on the Nexus 5.
+// See also: https://skia-review.googlesource.com/c/skia/+/148480
+#ifdef IMPELLER_TARGET_OPENGLES
+#define POW(x, y) exp2(y* log2(x))
+#else
+#define POW(x, y) pow(x, y)
+#endif
+
+#endif  // MATH_GLSL_

engine/src/flutter/impeller/entity/shaders/rrect_blur.frag

@@ -9,6 +9,7 @@
 precision highp float;
 
 #include <impeller/gaussian.glsl>
+#include <impeller/math.glsl>
 #include <impeller/types.glsl>
 
 uniform FragInfo {
@@ -44,9 +45,9 @@ float computeErf7(float x) {
 
 // The length formula, but with an exponent other than 2
 float powerDistance(vec2 p) {
-  float xp = pow(p.x, frag_info.exponent);
+  float xp = POW(p.x, frag_info.exponent);
-  float yp = pow(p.y, frag_info.exponent);
+  float yp = POW(p.y, frag_info.exponent);
-  return pow(xp + yp, frag_info.exponentInv);
+  return POW(xp + yp, frag_info.exponentInv);
 }
 
 void main() {

engine/src/flutter/impeller/tools/malioc.json

@@ -4365,7 +4365,7 @@
           },
           "thread_occupancy": 100,
           "uniform_registers_used": 4,
-          "work_registers_used": 3
+          "work_registers_used": 2
         }
       }
     }