Added workaround for NVidia reporting compressed block size in bits.

Instead of bytes as per specification of glGetInternalformativ().

src/Magnum/AbstractTexture.cpp

@@ -199,10 +199,19 @@ void AbstractTexture::bindImplementationMulti(const GLint firstTextureUnit, Cont
 
 #ifndef MAGNUM_TARGET_GLES
 Int AbstractTexture::compressedBlockDataSize(const GLenum target, const TextureFormat format) {
+    return (Context::current()->state().texture->compressedBlockDataSizeImplementation)(target, format);
+}
+
+Int AbstractTexture::compressedBlockDataSizeImplementationDefault(const GLenum target, const TextureFormat format) {
     GLint value;
     glGetInternalformativ(target, GLenum(format), GL_TEXTURE_COMPRESSED_BLOCK_SIZE, 1, &value);
     return value;
 }
+
+Int AbstractTexture::compressedBlockDataSizeImplementationBitsWorkaround(const GLenum target, const TextureFormat format) {
+    /* NVidia (358.16) reports the value in bits instead of bytes */
+    return compressedBlockDataSizeImplementationDefault(target, format)/8;
+}
 #endif
 
 AbstractTexture::AbstractTexture(GLenum target): _target{target}, _flags{ObjectFlag::DeleteOnDestruction} {

src/Magnum/AbstractTexture.h

@@ -416,6 +416,11 @@ class MAGNUM_EXPORT AbstractTexture: public AbstractObject {
         GLenum _target;
 
     private:
+        #ifndef MAGNUM_TARGET_GLES
+        static Int MAGNUM_LOCAL compressedBlockDataSizeImplementationDefault(GLenum target, TextureFormat format);
+        static Int MAGNUM_LOCAL compressedBlockDataSizeImplementationBitsWorkaround(GLenum target, TextureFormat format);
+        #endif
+
         static void MAGNUM_LOCAL unbindImplementationDefault(GLint textureUnit);
         #ifndef MAGNUM_TARGET_GLES
         static void MAGNUM_LOCAL unbindImplementationMulti(GLint textureUnit);

src/Magnum/Implementation/TextureState.cpp

@@ -374,6 +374,15 @@ TextureState::TextureState(Context& context, std::vector<std::string>& extension
         setMaxAnisotropyImplementation = &AbstractTexture::setMaxAnisotropyImplementationExt;
     } else setMaxAnisotropyImplementation = &AbstractTexture::setMaxAnisotropyImplementationNoOp;
 
+    #ifndef MAGNUM_TARGET_GLES
+    /* NVidia workaround for compressed block data size implementation */
+    if((context.detectedDriver() & Context::DetectedDriver::NVidia) &&
+        !context.isDriverWorkaroundDisabled("nv-compressed-block-size-in-bits"))
+        compressedBlockDataSizeImplementation = &AbstractTexture::compressedBlockDataSizeImplementationBitsWorkaround;
+    else
+        compressedBlockDataSizeImplementation = &AbstractTexture::compressedBlockDataSizeImplementationDefault;
+    #endif
+
     /* Resize bindings array to hold all possible texture units */
     glGetIntegerv(GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS, &maxTextureUnits);
     CORRADE_INTERNAL_ASSERT(maxTextureUnits > 0);

src/Magnum/Implementation/TextureState.h

@@ -45,6 +45,7 @@ struct TextureState {
 
     void reset();
 
+    Int(*compressedBlockDataSizeImplementation)(GLenum, TextureFormat);
     void(*unbindImplementation)(GLint);
     void(*bindMultiImplementation)(GLint, Containers::ArrayView<AbstractTexture* const>);
     void(AbstractTexture::*createImplementation)();

src/Magnum/Implementation/driverSpecific.cpp

@@ -52,6 +52,10 @@ namespace {
            newer GLSL version. */
         "no-layout-qualifiers-on-old-glsl",
 
+        /* NVidia drivers (358.16) report compressed block size from internal
+           format query in bits instead of bytes */
+        "nv-compressed-block-size-in-bits",
+
         /* NVidia drivers (358.16) report different compressed image size for
            cubemaps based on whether the texture is immutable or not and not
            based on whether I'm querying all faces (ARB_DSA) or a single face