Added ES2-only support for GL_LUMINANCE{,_ALPHA}.

It is deprecated, but many ES2 implementations don't support
EXT_texture_rg, thus this is the only way to have single- and
two-component textures without wasting precious memory. The enum value
isn't exposed on desktop OpenGL and ES3. GL_ALPHA is not supported, as
it doesn't bring any new functionality (it is also single-component
and thus can be interchanged with GL_LUMINANCE).

doc/unsupported.dox

@@ -29,8 +29,6 @@ add any performance gains, is not supported in %Magnum.
 
 @section unsupported-features Unsupported features
 
-- Luminance texture formats (OpenGL ES) are not supported, as they are
-  deprecated in OpenGL ES 3.0 and not present in core desktop OpenGL.
 - Fixed precision data types (OpenGL ES) are not supported, as they occupy the
   same memory as floats and they aren't faster than floats on current hardware
   anymore.

src/AbstractImage.cpp

@@ -99,11 +99,17 @@ std::size_t AbstractImage::pixelSize(ImageFormat format, ImageType type) {
         case ImageFormat::Blue:
         case ImageFormat::GreenInteger:
         case ImageFormat::BlueInteger:
+        #endif
+        #ifdef MAGNUM_TARGET_GLES2
+        case ImageFormat::Luminance:
         #endif
             return 1*size;
         case ImageFormat::RG:
         #ifndef MAGNUM_TARGET_GLES2
         case ImageFormat::RGInteger:
+        #endif
+        #ifdef MAGNUM_TARGET_GLES2
+        case ImageFormat::LuminanceAlpha:
         #endif
             return 2*size;
         case ImageFormat::RGB:

src/ImageFormat.cpp

@@ -37,7 +37,13 @@ Debug operator<<(Debug debug, ImageFormat value) {
         _c(Green)
         _c(Blue)
         #endif
+        #ifdef MAGNUM_TARGET_GLES2
+        _c(Luminance)
+        #endif
         _c(RG)
+        #ifdef MAGNUM_TARGET_GLES2
+        _c(LuminanceAlpha)
+        #endif
         _c(RGB)
         _c(RGBA)
         #ifndef MAGNUM_TARGET_GLES

src/ImageFormat.h

@@ -68,8 +68,18 @@ enum class ImageFormat: GLenum {
      *      available in OpenGL ES.
      */
     Blue = GL_BLUE,
+    #endif
 
-    /** @todo GL_ALPHA? */
+    #if defined(MAGNUM_TARGET_GLES2) || defined(DOXYGEN_GENERATING_OUTPUT)
+    /**
+     * Floating-point luminance channel. The value is used for all RGB
+     * channels.
+     * @deprecated Included for compatibility reasons only, use
+     *      @ref Magnum::ImageFormat "ImageFormat::Red" instead.
+     * @requires_gles20 Not available in ES 3.0 or desktop OpenGL. Use
+     *      @ref Magnum::ImageFormat "ImageFormat::Red" instead.
+     */
+    Luminance = GL_LUMINANCE,
     #endif
 
     /**
@@ -84,6 +94,18 @@ enum class ImageFormat: GLenum {
     RG = GL_RG_EXT,
     #endif
 
+    #if defined(MAGNUM_TARGET_GLES2) || defined(DOXYGEN_GENERATING_OUTPUT)
+    /**
+     * Floating-point luminance and alpha channel. First value is used for all
+     * RGB channels, second value is used for alpha channel.
+     * @deprecated Included for compatibility reasons only, use
+     *      @ref Magnum::ImageFormat "ImageFormat::RG" instead.
+     * @requires_gles20 Not available in ES 3.0 or desktop OpenGL. Use
+     *      @ref Magnum::ImageFormat "ImageFormat::RG" instead.
+     */
+    LuminanceAlpha = GL_LUMINANCE_ALPHA,
+    #endif
+
     /**
      * Floating-point RGB.
      * @requires_gl Can't be used for framebuffer reading in OpenGL ES.

src/TextureFormat.h

@@ -475,7 +475,32 @@ enum class TextureFormat: GLenum {
      * @requires_gl Packed 8bit types are not available in OpenGL ES.
      */
     R3B3G2 = GL_R3_G3_B2,
+    #endif
+
+    #if defined(MAGNUM_TARGET_GLES2) || defined(DOXYGEN_GENERATING_OUTPUT)
+    /**
+     * Luminance, normalized unsigned, single value used for all RGB channels.
+     * Size implementation-dependent.
+     * @deprecated Included for compatibility reasons only, use
+     *      @ref Magnum::TextureFormat "TextureFormat::R8" instead.
+     * @requires_gles20 Not available in ES 3.0 or desktop OpenGL. Use
+     *      @ref Magnum::TextureFormat "TextureFormat::R8" instead.
+     */
+    Luminance = GL_LUMINANCE,
+
+    /**
+     * Floating-point luminance and alpha channel. First value is used for all
+     * RGB channels, second value is used for alpha channel. Size
+     * implementation-dependent.
+     * @deprecated Included for compatibility reasons only, use
+     *      @ref Magnum::TextureFormat "TextureFormat::RG" instead.
+     * @requires_gles20 Not available in ES 3.0 or desktop OpenGL. Use
+     *      @ref Magnum::TextureFormat "TextureFormat::RG" instead.
+     */
+    LuminanceAlpha = GL_LUMINANCE_ALPHA,
+    #endif
 
+    #ifndef MAGNUM_TARGET_GLES
     /**
      * RGB, each component normalized unsigned 4bit.
      * @requires_gl Packed 12bit types are not available in OpenGL ES.