Fix precision issues in vertex shaders

Restore conditional that makes mediump declaration only apply to fragment shaders
This was erroneously removed to make the Lanczos3 shader link properly, but it resulted in vertex shaders incorrectly defaulting to mediump
Add highp precision declaration to Lanczos3 fragment shader so that it can link

shader/common.h

@@ -1,6 +1,9 @@
 #ifdef GLSLES
 
+#ifdef FRAGMENT_SHADER
+/* Only the fragment shader has no default float precision */
 precision mediump float;
+#endif
 
 #else
 

shader/lanczos3.frag

@@ -2,6 +2,7 @@
 // Copyright 2020 Lilian Gimenez (Sentmoraap).
 // MIT license.
 
+precision highp float;
 uniform sampler2D texture;
 uniform vec2 sourceSize;
 uniform vec2 texSizeInv;