GL: pack the Mesh members better.

Saves 8 bytes (104 -> 96 bytes), which were before wasted on 4
byte padding before the 8-byte _indexBufferOffset member and 4 bytes
after the _indexBuffer, which is new there due to the Buffer being 8
instead of 12 bytes now.

On ES2 there's three 32-bit members less, which means this change only
moved the 4-byte after _indexBuffer to after _indexType, i.e. 88 bytes
before and 88 now as well.

src/Magnum/GL/Mesh.cpp

@@ -344,7 +344,7 @@ Mesh::Mesh(Mesh&& other) noexcept: _id(other._id), _primitive(other._primitive),
     #ifndef MAGNUM_TARGET_GLES2
     _indexStart(other._indexStart), _indexEnd(other._indexEnd),
     #endif
-    _indexBufferOffset(other._indexBufferOffset), _indexOffset(other._indexOffset), _indexType(other._indexType), _indexBuffer{Utility::move(other._indexBuffer)}
+    _indexType(other._indexType), _indexBufferOffset(other._indexBufferOffset), _indexOffset(other._indexOffset), _indexBuffer{Utility::move(other._indexBuffer)}
 {
     if(_constructed || other._constructed)
         Context::current().state().mesh.moveConstructImplementation(*this, Utility::move(other));
@@ -368,8 +368,8 @@ Mesh& Mesh::operator=(Mesh&& other) noexcept {
     swap(_indexEnd, other._indexEnd);
     #endif
     swap(_indexBufferOffset, other._indexBufferOffset);
-    swap(_indexOffset, other._indexOffset);
     swap(_indexType, other._indexType);
+    swap(_indexOffset, other._indexOffset);
     swap(_indexBuffer, other._indexBuffer);
 
     if(_constructed || other._constructed)

src/Magnum/GL/Mesh.h

@@ -1432,8 +1432,9 @@ class MAGNUM_GL_EXPORT Mesh: public AbstractObject {
         UnsignedInt _baseInstance{};
         UnsignedInt _indexStart{}, _indexEnd{};
         #endif
-        GLintptr _indexBufferOffset{}, _indexOffset{};
         MeshIndexType _indexType{};
+        /* 4 bytes free on ES2 */
+        GLintptr _indexBufferOffset{}, _indexOffset{};
         Buffer _indexBuffer{NoCreate};
 
         /* Storage for either std::vector<AttributeLayout> (in case of no VAOs)