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/nodes_shader/opengl_vertex.glsl

@@ -16,7 +16,10 @@ varying vec3 vPosition;
 // precision must be considered).
 varying vec3 worldPosition;
 varying lowp vec4 varColor;
-varying mediump vec2 varTexCoord;
+// The centroid keyword ensures that after interpolation the texture coordinates
+// lie within the same bounds when MSAA is en- and disabled.
+// This fixes the stripes problem with nearest-neighbour textures and MSAA.
+centroid varying mediump vec2 varTexCoord;
 varying vec3 eyeVec;
 
 // Color of the light emitted by the light sources.
@@ -142,7 +145,7 @@ void main(void)
 	vec4 color;
 	// The alpha gives the ratio of sunlight in the incoming light.
 	float nightRatio = 1.0 - inVertexColor.a;
-	color.rgb = inVertexColor.rgb * (inVertexColor.a * dayLight.rgb + 
+	color.rgb = inVertexColor.rgb * (inVertexColor.a * dayLight.rgb +
 		nightRatio * artificialLight.rgb) * 2.0;
 	color.a = 1.0;