Text: unconditionally use TextureFormat::Luminance in GlyphCache on ES2.

Because now with the generic formats all images that are in
PixelFormat::R8Unorm are translated to GL::PixelFormat::Luminance on ES2
and WebGL 1. The DistanceFieldGlyphCache still has the original, but
that one didn't really work there in the first place. That'll get
patched later.

doc/changelog.dox

@@ -138,6 +138,14 @@ See also:
     now expect that the mesh is indexed (instead of silently not doing
     anything)
 
+@subsubsection changelog-latest-changes-text Text library
+
+-   The @ref Text::GlyphCache now unconditionally uses
+    @ref GL::TextureFormat::Luminance on ES2 and WebGL 1, instead of trying
+    @ref GL::TextureFormat::Red there first. That's done for consistency with
+    @ref GL::pixelFormat(), which unconditionally returns
+    @ref GL::PixelFormat::Luminance for @ref PixelFormat::R8Unorm.
+
 @subsubsection changelog-latest-changes-plugins Plugins
 
 -   @ref Trade::TgaImporter "TgaImporter" and

src/Magnum/Text/GlyphCache.cpp

@@ -46,15 +46,8 @@ GlyphCache::GlyphCache(const Vector2i& originalSize, const Vector2i& size, const
     MAGNUM_ASSERT_GL_EXTENSION_SUPPORTED(GL::Extensions::ARB::texture_rg);
     #endif
 
-    /** @todo Is there any better way to select proper sized/unsized format on ES2? */
     #ifndef MAGNUM_TARGET_GLES2
     const GL::TextureFormat internalFormat = GL::TextureFormat::R8;
-    #elif !defined(MAGNUM_TARGET_WEBGL)
-    GL::TextureFormat internalFormat;
-    if(GL::Context::current().isExtensionSupported<GL::Extensions::EXT::texture_rg>()) {
-        internalFormat = GL::Context::current().isExtensionSupported<GL::Extensions::EXT::texture_storage>() ?
-            GL::TextureFormat::R8 : GL::TextureFormat::Red;
-    } else internalFormat = GL::TextureFormat::Luminance;
     #else
     const GL::TextureFormat internalFormat = GL::TextureFormat::Luminance;
     #endif

src/Magnum/Text/GlyphCache.h

@@ -81,9 +81,13 @@ class MAGNUM_TEXT_EXPORT GlyphCache {
          * @brief Constructor
          *
          * Sets internal texture format to red channel only. On desktop OpenGL
-         * requires @extension{ARB,texture_rg} (also part of OpenGL ES 3.0), in
-         * ES2 uses @extension{EXT,texture_rg}, if available, or
-         * @ref GL::TextureFormat::Luminance as fallback.
+         * requires @extension{ARB,texture_rg} (also part of OpenGL ES 3.0 and
+         * WebGL 2), on ES2 unconditionally uses @ref GL::TextureFormat::Luminance.
+         * This is done for consistency with @ref GL::pixelFormat(), which
+         * unconditionally returns @ref GL::PixelFormat::Luminance for
+         * @ref PixelFormat::R8Unorm. See
+         * @ref GlyphCache(GL::TextureFormat, const Vector2i&, const Vector2i&)
+         * for an alternative.
          */
         explicit GlyphCache(const Vector2i& originalSize, const Vector2i& size, const Vector2i& padding);