renderengine: avoid divide-by-zero in shaders

We have

   max(1e-6, color.y)

to make sure the divisor is non-zero.  But when mediump means
half-precision floats, 1e-6 is smaller than the minimum positive
normal number.  The line might have no effect at all.

This is noticed when looking into the referenced bug.

Bug: 116864006
Test: manual
Change-Id: Ic39209e19ceeba6f51688dc9549746bf0f6a98e1
diff --git a/services/surfaceflinger/RenderEngine/gl/ProgramCache.cpp b/services/surfaceflinger/RenderEngine/gl/ProgramCache.cpp
index bc7d111..c118248 100644
--- a/services/surfaceflinger/RenderEngine/gl/ProgramCache.cpp
+++ b/services/surfaceflinger/RenderEngine/gl/ProgramCache.cpp
@@ -316,7 +316,7 @@
 
                             // scale [0.0, maxInLumi] to [0.0, maxOutLumi]
                             if (maxInLumi <= maxOutLumi) {
-                                nits *= maxOutLumi / maxInLumi;
+                                return color * (maxOutLumi / maxInLumi);
                             } else {
                                 // three control points
                                 const float x0 = 10.0;
@@ -337,7 +337,7 @@
                                 if (nits < x0) {
                                     // scale [0.0, x0] to [0.0, y0] linearly
                                     float slope = y0 / x0;
-                                    nits *= slope;
+                                    return color * slope;
                                 } else if (nits < x1) {
                                     // scale [x0, x1] to [y0, y1] linearly
                                     float slope = (y1 - y0) / (x1 - x0);
@@ -355,7 +355,8 @@
                                 }
                             }
 
-                            return color * (nits / max(1e-6, color.y));
+                            // color.y is greater than x0 and is thus non-zero
+                            return color * (nits / color.y);
                         }
                     )__SHADER__";
                     break;
@@ -386,7 +387,7 @@
                     if (nits <= x0) {
                         // scale [0.0, x0] to [0.0, y0] linearly
                         const float slope = y0 / x0;
-                        nits *= slope;
+                        return color * slope;
                     } else if (nits <= x1) {
                         // scale [x0, x1] to [y0, y1] using a curve
                         float t = (nits - x0) / (x1 - x0);
@@ -401,7 +402,8 @@
                         nits = (1.0 - t) * (1.0 - t) * y2 + 2.0 * (1.0 - t) * t * c3 + t * t * y3;
                     }
 
-                    return color * (nits / max(1e-6, color.y));
+                    // color.y is greater than x0 and is thus non-zero
+                    return color * (nits / color.y);
                 }
             )__SHADER__";
             break;