render: Bypass texture rendering when a model uses the null texture.

* data/shaders/default-fragment.glsl (use_texture): New uniform.
  (main): Skip texture rendering when 'use_texture' is false.
* sly/render/model.scm (draw-model): Set 'use_texture' uniform before
  rendering.
* sly/render/shader.scm (load-default-shader): Register 'use_texture'
  uniform.

data/shaders/default-fragment.glsl

@@ -3,7 +3,12 @@
 in vec2 frag_tex;
 uniform vec4 color;
 uniform sampler2D color_texture;
+uniform bool use_texture;
 
 void main (void) {
+  if(use_texture) {
     gl_FragColor = texture2D(color_texture, frag_tex) * color;
+  } else {
+    gl_FragColor = color;
+  }
 }

sly/render/model.scm

@@ -155,6 +155,7 @@ CONTEXT."
               ;; TODO: Support user-defined uniforms.
               (uniform-set! shader "mvp" (render-context-transform context))
               (uniform-set! shader "color" color)
+              (uniform-set! shader "use_texture" (not (texture-null? texture)))
               (glDrawElements (begin-mode triangles)
                               (mesh-length mesh)
                               (data-type unsigned-int)

sly/render/shader.scm

@@ -305,7 +305,7 @@ VERTEX-SHADER and FRAGMENT-SHADER."
                      "/shaders/default-vertex.glsl")
       (string-append %pkgdatadir
                      "/shaders/default-fragment.glsl")
-      '("mvp" "color")
+      '("mvp" "color" "use_texture")
       '("position" "tex")))))
 
 ;;;