Minor cleanup in image and texture format enums.

Removed hierarchic and confusing `#ifdef`s.

src/ImageFormat.h

@@ -449,11 +449,13 @@ enum class ImageType: GLenum {
      * @requires_gl30 %Extension @extension{EXT,packed_depth_stencil}
      * @requires_gles30 For texture data only, extension @es_extension{OES,packed_depth_stencil}.
      */
-    #ifdef MAGNUM_TARGET_GLES2
-    UnsignedInt248 = GL_UNSIGNED_INT_24_8_OES
-    #else
+    #ifndef MAGNUM_TARGET_GLES2
     UnsignedInt248 = GL_UNSIGNED_INT_24_8,
+    #else
+    UnsignedInt248 = GL_UNSIGNED_INT_24_8_OES,
+    #endif
 
+    #ifndef MAGNUM_TARGET_GLES2
     /**
      * Float + unsigned int, depth component 32bit float, 24bit gap, stencil
      * index 8bit.

src/TextureFormat.h

@@ -147,6 +147,7 @@ enum class TextureFormat: GLenum {
      * @requires_gles30 Only unsigned formats are available in OpenGL ES 2.0.
      */
     RGBA8Snorm = GL_RGBA8_SNORM,
+    #endif
 
     #ifndef MAGNUM_TARGET_GLES
     /**
@@ -212,6 +213,7 @@ enum class TextureFormat: GLenum {
     RGBA16Snorm = GL_RGBA16_SNORM,
     #endif
 
+    #ifndef MAGNUM_TARGET_GLES2
     /**
      * Red component, non-normalized unsigned byte.
      * @requires_gl30 %Extension @extension{ARB,texture_rg} and @extension{EXT,texture_integer}