TgaImporter: return Luminance if EXT_texture_rg is not supported in ES.

Also documented the behavior.

src/Plugins/TgaImporter/TgaImporter.cpp

@@ -33,6 +33,8 @@
 
 #ifdef MAGNUM_TARGET_GLES
 #include <algorithm>
+#include <Context.h>
+#include <Extensions.h>
 #include <Swizzle.h>
 #endif
 
@@ -117,7 +119,12 @@ ImageData2D* TgaImporter::doImage2D(UnsignedInt) {
 
     /* Grayscale */
     } else if(header.imageType == 3) {
+        #ifdef MAGNUM_TARGET_GLES
+        format = Context::current() && Context::current()->isExtensionSupported<Extensions::GL::EXT::texture_rg>() ?
+            ImageFormat::Red : ImageFormat::Luminance;
+        #else
         format = ImageFormat::Red;
+        #endif
         if(header.bpp != 8) {
             Error() << "Trade::TgaImporter::TgaImporter::image2D(): unsupported grayscale bits-per-pixel:" << header.bpp;
             return nullptr;

src/Plugins/TgaImporter/TgaImporter.h

@@ -46,6 +46,16 @@ namespace Magnum { namespace Trade {
 @brief TGA image importer
 
 Supports uncompressed BGR, BGRA or grayscale images with 8 bits per channel.
+
+The images are imported with @ref ImageType "ImageType::UnsignedByte" and
+@ref ImageFormat "ImageFormat::BGR", @ref ImageFormat "ImageFormat::BGRA" or
+@ref ImageFormat "ImageFormat::Red" respectively. Grayscale images require
+extension @extension{ARB,texture_rg}.
+
+In OpenGL ES BGR and BGRA images are converted to @ref ImageFormat "ImageFormat::RGB"
+and @ref ImageFormat "ImageFormat::RGBA". In OpenGL ES 2.0, if @es_extension{EXT,texture_rg}
+is not supported, grayscale images use @ref ImageFormat "ImageFormat::Luminance"
+instead of @ref ImageFormat "ImageFormat::Red".
 */
 class MAGNUM_TRADE_TGAIMPORTER_EXPORT TgaImporter: public AbstractImporter {
     public: