Add support for rendering tinted artwork.

glsl/model.vert

@@ -7,6 +7,7 @@ uniform mat4 TransformMatrix;
 attribute vec4 aVertexPosition;
 attribute vec4 aVertexTexCoord;
 attribute vec2 aVertexTexMetadata;
+attribute vec3 aVertexTint;
 varying vec4 vTexCoord;
 varying vec4 vChannelMask;
 varying vec4 vNormalsMask;
@@ -14,6 +15,7 @@ varying vec4 vNormalsMask;
 in vec4 aVertexPosition;
 in vec4 aVertexTexCoord;
 in vec2 aVertexTexMetadata;
+in vec3 aVertexTint;
 out vec4 vTexCoord;
 out vec4 vChannelMask;
 out vec4 vNormalsMask;