replace gl_FragColor with frag_color output vector in fragment shaders

runtime/shaders/basic.f.glsl

@@ -2,7 +2,9 @@
 
 uniform vec4 color;
 
+out vec4 frag_color;
+
 void main(void)
 {
-    gl_FragColor = color;
+    frag_color = color;
 }

runtime/shaders/text.f.glsl

@@ -7,7 +7,9 @@ in vec2 texture_coord_v;
 uniform sampler2D texture;
 uniform vec4 color;
 
+out vec4 frag_color;
+
 void main(void)
 {
-    gl_FragColor = vec4(1, 1, 1, texture2D(texture, texture_coord_v).a) * color;
+    frag_color = vec4(1, 1, 1, texture2D(texture, texture_coord_v).a) * color;
 }