Use `size >> mipLevel` to compute image size at given mip level.

This is so awesome trick.

src/AbstractTexture.cpp

@@ -683,10 +683,8 @@ void AbstractTexture::storageImplementationFallback(const GLenum target, const G
     const ImageFormat format = imageFormatForInternalFormat(internalFormat);
     const ImageType type = imageTypeForInternalFormat(internalFormat);
 
-    auto levelSize = size;
     for(GLsizei level = 0; level != levels; ++level) {
-        (this->*image1DImplementation)(target, level, internalFormat, levelSize, format, type, nullptr);
-        levelSize = Math::max(Math::Vector<1, GLsizei>(1), levelSize/2);
+        (this->*image1DImplementation)(target, level, internalFormat, Math::max(Math::Vector<1, GLsizei>(1), size >> level), format, type, nullptr);
     }
 }
 
@@ -720,15 +718,12 @@ void AbstractTexture::storageImplementationFallback(const GLenum target, const G
     if(target == GL_TEXTURE_2D)
     #endif
     {
-        Vector2i levelSize = size;
         for(GLsizei level = 0; level != levels; ++level) {
-            (this->*image2DImplementation)(target, level, internalFormat, levelSize, format, type, nullptr);
-            levelSize = Math::max(Vector2i(1), levelSize/2);
+            (this->*image2DImplementation)(target, level, internalFormat, Math::max(Vector2i(1), size >> level), format, type, nullptr);
         }
 
     /* Cube map additionally needs to specify all faces */
     } else if(target == GL_TEXTURE_CUBE_MAP) {
-        Vector2i levelSize = size;
         for(GLsizei level = 0; level != levels; ++level) {
             for(GLenum face: {GL_TEXTURE_CUBE_MAP_POSITIVE_X,
                               GL_TEXTURE_CUBE_MAP_NEGATIVE_X,
@@ -736,17 +731,14 @@ void AbstractTexture::storageImplementationFallback(const GLenum target, const G
                               GL_TEXTURE_CUBE_MAP_NEGATIVE_Y,
                               GL_TEXTURE_CUBE_MAP_POSITIVE_Z,
                               GL_TEXTURE_CUBE_MAP_NEGATIVE_Z})
-                (this->*image2DImplementation)(face, level, internalFormat, levelSize, format, type, nullptr);
-            levelSize = Math::max(Vector2i(1), levelSize/2);
+                (this->*image2DImplementation)(face, level, internalFormat, Math::max(Vector2i(1), size >> level), format, type, nullptr);
         }
 
     #ifndef MAGNUM_TARGET_GLES
     /* Array texture is not scaled in "layer" dimension */
     } else if(target == GL_TEXTURE_1D_ARRAY) {
-        Vector2i levelSize = size;
         for(GLsizei level = 0; level != levels; ++level) {
-            (this->*image2DImplementation)(target, level, internalFormat, levelSize, format, type, nullptr);
-            levelSize.x() = Math::max(1, levelSize.x()/2);
+            (this->*image2DImplementation)(target, level, internalFormat, Math::max(Vector2i(1), size >> level), format, type, nullptr);
         }
     #endif
 
@@ -785,10 +777,8 @@ void AbstractTexture::storageImplementationFallback(GLenum target, GLsizei level
     if(target == GL_TEXTURE_3D_OES)
     #endif
     {
-        Vector3i levelSize = size;
         for(GLsizei level = 0; level != levels; ++level) {
-            (this->*image3DImplementation)(target, level, internalFormat, levelSize, format, type, nullptr);
-            levelSize = Math::max(Vector3i(1), levelSize/2);
+            (this->*image3DImplementation)(target, level, internalFormat, Math::max(Vector3i(1), size >> level), format, type, nullptr);
         }
 
     #ifndef MAGNUM_TARGET_GLES2
@@ -800,10 +790,8 @@ void AbstractTexture::storageImplementationFallback(GLenum target, GLsizei level
     else if(target == GL_TEXTURE_2D_ARRAY)
     #endif
     {
-        Vector3i levelSize = size;
         for(GLsizei level = 0; level != levels; ++level) {
-            (this->*image3DImplementation)(target, level, internalFormat, levelSize, format, type, nullptr);
-            levelSize.xy() = Math::max(Vector2i(1), levelSize.xy()/2);
+            (this->*image3DImplementation)(target, level, internalFormat, Math::max(Vector3i(1), size >> level), format, type, nullptr);
         }
     #endif