Fix large radius RR clipping issue on certain devices

bug:16804363

Since dist can be in the 1000s of pixels, and length() may square it
in its current precision, scale the value down significantly first,
since final precision isn't very important.

Change-Id: Id20f7a49d6171355c8e242442c2b5083f746dca3

libs/hwui/ProgramCache.cpp

@@ -334,8 +334,10 @@ const char* gFS_Main_ApplyColorOp[3] = {
 const char* gFS_Main_FragColor_HasRoundRectClip =
         "    mediump vec2 fragToLT = roundRectInnerRectLTRB.xy - roundRectPos;\n"
         "    mediump vec2 fragFromRB = roundRectPos - roundRectInnerRectLTRB.zw;\n"
-        "    mediump vec2 dist = max(max(fragToLT, fragFromRB), vec2(0.0, 0.0));\n"
-        "    mediump float linearDist = roundRectRadius - length(dist);\n"
+
+        // divide + multiply by 128 to avoid falling out of range in length() function
+        "    mediump vec2 dist = max(max(fragToLT, fragFromRB), vec2(0.0, 0.0)) / 128.0;\n"
+        "    mediump float linearDist = roundRectRadius - (length(dist) * 128.0);\n"
         "    gl_FragColor *= clamp(linearDist, 0.0, 1.0);\n";
 
 const char* gFS_Main_DebugHighlight =