Support for setting textures as uniforms in AbstractShaderProgram.

Also updated Texture documentation.

src/AbstractShaderProgram.h

@@ -22,6 +22,7 @@
 #include <map>
 
 #include "Shader.h"
+#include "Texture.h"
 
 namespace Magnum {
 
@@ -197,6 +198,11 @@ class AbstractShaderProgram {
             glUniformMatrix4fv(location, 1, GL_FALSE, value.data());
         }
 
+        /** @copydoc setUniform(GLint, GLint) */
+        void setUniform(GLint location, const AbstractTexture* value) {
+            setUniform(location, value->layer());
+        }
+
     private:
         enum State {
             Initialized,

src/AbstractTexture.h

@@ -127,6 +127,9 @@ class AbstractTexture {
             glDeleteTextures(1, &texture);
         }
 
+        /** @brief Texture layer */
+        inline GLint layer() const { return _layer; }
+
         /** @brief Bind texture for usage / rendering */
         inline void bind() const {
             glActiveTexture(GL_TEXTURE0 + _layer);

src/Texture.h

@@ -33,9 +33,11 @@ subclassing and setting texture data from e.g. constructor with setData().
 setMagnificationFilter() after creating the texture, otherwise it will be
 unusable.
 
-The texture is bound via bind() and setting texture uniform on the shader to
+The texture is bound via bind() and setting texture uniform on the shader to the
-desired texture layer. In shader, the texture is used via @c sampler1D,
+texture (see AbstractShaderProgram::setUniform(GLint, const AbstractTexture*)).
-@c sampler2D or @c sampler3D depending on dimension count.
+In shader, the texture is used via @c sampler1D, @c sampler2D or @c sampler3D
+depending on dimension count. Note that you can have more than one texture bound
+to the shader - the only requirement is to have each texture in another layer.
 
 @section RectangleTextures Rectangle textures