Fix OpenGL 2D custom shader crash on newer Android devices

modules/juce_opengl/opengl/juce_OpenGLGraphicsContext.cpp

@@ -1853,7 +1853,7 @@ struct CustomProgram  : public ReferenceCountedObject,
 OpenGLGraphicsContextCustomShader::OpenGLGraphicsContextCustomShader (const String& fragmentShaderCode)
     : code (String (JUCE_DECLARE_VARYING_COLOUR
                     JUCE_DECLARE_VARYING_PIXELPOS
-                    "\n" JUCE_MEDIUMP " float pixelAlpha = frontColour.a;\n") + fragmentShaderCode),
+                    "\n#define pixelAlpha frontColour.a\n") + fragmentShaderCode),
       hashName (String::toHexString (fragmentShaderCode.hashCode64()) + "_shader")
 {
 }