Use highp precision when available for GLES

src/nanovg_gl.h

@@ -384,11 +384,15 @@ static int glnvg__renderCreate(void* uptr)
 		"#define NANOVG_GL3 1\n";
 #elif defined NANOVG_GLES2
 		"#version 100\n"
-		"precision mediump float;\n"
+		"#ifdef GL_FRAGMENT_PRECISION_HIGH\n"
+		" precision highp float;\n"
+		"#else\n"
+		" precision mediump float;\n"
+		"#endif\n"
 		"#define NANOVG_GL2 1\n";
 #elif defined NANOVG_GLES3
 		"#version 300 es\n"
-		"precision mediump float;\n"
+		"precision highp float;\n"
 		"#define NANOVG_GL3 1\n";
 #endif