Ability to reset BufferImage formats without reallocating the buffer.

Will be used in various image getters to avoid overwriting previous data
in combination with pixel storage skip parameters.

src/Magnum/BufferImage.cpp

@@ -28,20 +28,27 @@
 namespace Magnum {
 
 #ifndef MAGNUM_TARGET_GLES2
-template<UnsignedInt dimensions> BufferImage<dimensions>::BufferImage(const PixelStorage storage, const PixelFormat format, const PixelType type, const VectorTypeFor<dimensions, Int>& size, Containers::ArrayView<const void> const data, const BufferUsage usage): _storage{storage}, _format{format}, _type{type}, _size{size}, _buffer{Buffer::TargetHint::PixelPack} {
+template<UnsignedInt dimensions> BufferImage<dimensions>::BufferImage(const PixelStorage storage, const PixelFormat format, const PixelType type, const VectorTypeFor<dimensions, Int>& size, Containers::ArrayView<const void> const data, const BufferUsage usage): _storage{storage}, _format{format}, _type{type}, _size{size}, _buffer{Buffer::TargetHint::PixelPack}, _dataSize{data.size()} {
     CORRADE_ASSERT(Implementation::imageDataSize(*this) <= data.size(), "BufferImage::BufferImage(): bad image data size, got" << data.size() << "but expected at least" << Implementation::imageDataSize(*this), );
     _buffer.setData(data, usage);
 }
 
-template<UnsignedInt dimensions> BufferImage<dimensions>::BufferImage(const PixelStorage storage, const PixelFormat format, const PixelType type): _storage{storage}, _format{format}, _type{type}, _buffer{Buffer::TargetHint::PixelPack} {}
+template<UnsignedInt dimensions> BufferImage<dimensions>::BufferImage(const PixelStorage storage, const PixelFormat format, const PixelType type): _storage{storage}, _format{format}, _type{type}, _buffer{Buffer::TargetHint::PixelPack}, _dataSize{0} {}
 
 template<UnsignedInt dimensions> void BufferImage<dimensions>::setData(const PixelStorage storage, const PixelFormat format, const PixelType type, const VectorTypeFor<dimensions, Int>& size, Containers::ArrayView<const void> const data, const BufferUsage usage) {
     _storage = storage;
     _format = format;
     _type = type;
     _size = size;
-    CORRADE_ASSERT(Implementation::imageDataSize(*this) <= data.size(), "BufferImage::setData(): bad image data size, got" << data.size() << "but expected at least" << Implementation::imageDataSize(*this), );
-    _buffer.setData(data, usage);
+
+    /* Keep the old storage if zero-sized nullptr buffer was passed */
+    if(data.data() == nullptr && data.size() == 0)
+        CORRADE_ASSERT(Implementation::imageDataSize(*this) <= _dataSize, "BufferImage::setData(): bad current storage size, got" << _dataSize << "but expected at least" << Implementation::imageDataSize(*this), );
+    else {
+        CORRADE_ASSERT(Implementation::imageDataSize(*this) <= data.size(), "BufferImage::setData(): bad image data size, got" << data.size() << "but expected at least" << Implementation::imageDataSize(*this), );
+        _buffer.setData(data, usage);
+        _dataSize = data.size();
+    }
 }
 
 template<UnsignedInt dimensions> CompressedBufferImage<dimensions>::CompressedBufferImage(

src/Magnum/BufferImage.h

@@ -148,6 +148,9 @@ template<UnsignedInt dimensions> class BufferImage {
             return Implementation::imageDataProperties<dimensions>(*this);
         }
 
+        /** @brief Currently allocated data size */
+        std::size_t dataSize() const { return _dataSize; }
+
         /** @brief Image buffer */
         Buffer& buffer() { return _buffer; }
 
@@ -160,7 +163,9 @@ template<UnsignedInt dimensions> class BufferImage {
          * @param data              Image data
          * @param usage             Image buffer usage
          *
-         * Updates the image buffer with given data.
+         * Updates the image buffer with given data. Passing `nullptr`
+         * zero-sized @p data will not reallocate current storage, but expects
+         * that current data size is large enough for the new parameters.
          * @see @ref Buffer::setData()
          * @todo Make it more flexible (usable with
          *      @extension{ARB,buffer_storage}, avoiding relocations...)
@@ -201,6 +206,7 @@ template<UnsignedInt dimensions> class BufferImage {
         PixelType _type;
         Math::Vector<Dimensions, Int> _size;
         Buffer _buffer;
+        std::size_t _dataSize;
 };
 
 /** @brief One-dimensional buffer image */