respect srgb gamma when building mips

src/core/SkPM4fPriv.h

Index: src/core/SkPM4fPriv.h
diff --git a/src/core/SkPM4fPriv.h b/src/core/SkPM4fPriv.h
index bf5ff5a1ad44b4914caf36d005653dad4e20c5da..12bf7d0f36bb2b8bd608ebf56ea12e11e3862ca5 100644
--- a/src/core/SkPM4fPriv.h
+++ b/src/core/SkPM4fPriv.h
@@ -52,6 +52,48 @@ static inline float linear_to_srgb(float x) {
     return sqrtf(x);
 }
 
+static void assert_unit(float x) {
+    SkASSERT(x >= 0 && x <= 1);
+}
+
+static inline float exact_srgb_to_linear(float x) {
+    assert_unit(x);
+    float linear;
+    if (x <= 0.04045) {
+        linear = x / 12.92f;
+    } else {
+        linear = powf((x + 0.055f) / 1.055f, 2.4f);
+    }
+    assert_unit(linear);
+    return linear;
+}
+
+static inline float exact_linear_to_srgb(float x) {
+    assert_unit(x);
+    float srgb;
+    if (x <= 0.0031308f) {
+        srgb = x * 12.92f;
+    } else {
+        srgb = 1.055f * powf(x, 0.41666667f) - 0.055f;
+    }
+    assert_unit(srgb);
+    return srgb;
+}
+
+static inline Sk4f exact_srgb_to_linear(const Sk4f& x) {
+    Sk4f linear(exact_srgb_to_linear(x[0]),
+                exact_srgb_to_linear(x[1]),
+                exact_srgb_to_linear(x[2]), 1);
+    return set_alpha(linear, get_alpha(x));
+}
+
+static inline Sk4f exact_linear_to_srgb(const Sk4f& x) {
+    Sk4f srgb(exact_linear_to_srgb(x[0]),
+              exact_linear_to_srgb(x[1]),
+              exact_linear_to_srgb(x[2]), 1);
+    return set_alpha(srgb, get_alpha(x));
+}
+
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 static inline Sk4f Sk4f_fromL32(uint32_t src) {
@@ -77,4 +119,11 @@ static inline uint32_t Sk4f_toS32(const Sk4f& x4) {
     return to_4b(linear_to_srgb(x4) * Sk4f(255) + Sk4f(0.5f));
 }
 
+static inline Sk4f exact_Sk4f_fromS32(uint32_t src) {
+    return exact_srgb_to_linear(to_4f(src) * Sk4f(1.0f/255));
+}
+static inline uint32_t exact_Sk4f_toS32(const Sk4f& x4) {
+    return to_4b(exact_linear_to_srgb(x4) * Sk4f(255) + Sk4f(0.5f));
+}
+
 #endif