Fix MSAA stripes (#9247)

This only works when shaders are enabled.
The centroid varying avoids that the textures (which repeat themselves out of bounds) are sampled out of bounds in MSAA.
If MSAA (called FSAA in minetest) is disabled, the centroid keyword does nothing.

client/shaders/object_shader/opengl_vertex.glsl

@@ -7,7 +7,7 @@ varying vec3 vNormal;
 varying vec3 vPosition;
 varying vec3 worldPosition;
 varying lowp vec4 varColor;
-varying mediump vec2 varTexCoord;
+centroid varying mediump vec2 varTexCoord;
 
 varying vec3 eyeVec;
 varying float vIDiff;