use highp on iOS blending shader

src/app_shaders.cpp

@@ -95,14 +95,14 @@ void App::initShaders()
         "uniform int blend_mode;\n"
         "in mediump vec2 uv;\n"
 #ifdef __IOS__
-        "inout mediump vec4 frag;\n"
+        "inout highp vec4 frag;\n"
 #else
         "out mediump vec4 frag;\n"
 #endif
         SHADER_FUNCTION_BLEND
         "void main() {\n"
 #ifdef __IOS__
-        "   mediump vec4 bg = frag;\n"
+        "   highp vec4 bg = frag;\n"
 #else
         "   mediump vec4 bg = texture(tex_bg, uv);\n"
 #endif