Shader: Refine preprocessing on GLES platform

Don't globally set float precision to mediump, only fragment
shaders need that and defining it for vertex shaders causes
tilemap cracks.

Also manually define low precision for variables that hold
color / alpha values.

shader/common.h

@@ -0,0 +1,15 @@
+#ifdef GLSLES
+
+#ifdef FRAGMENT_SHADER
+/* Only the fragment shader has no default float precision */
+precision mediump float;
+#endif
+
+#else
+
+/* Desktop GLSL doesn't know about these */
+#define highp
+#define mediump
+#define lowp
+
+#endif