Remove temp buffer alloc get data when reading full texture (MonoGame#8590)

When GetData is called on an OpenGL Texture2D a temporary buffer is
allocated which causes heap allocations.
In scenarios where you request the entire texture (rect param is not
given, or rect param is of full extend (0,0,width,height). We do not
have to allocate a temporary buffer, and we can also skip Array.Copy in
this case.

### Description of Change
The proposed implementation checks for this specific condition, and then
proceeds to fill the given T[]Data directly with GetTexImage results.

---------

Co-authored-by: Tom Spilman <[email protected]>
Co-authored-by: Thomas Altenburger <[email protected]>
Co-authored-by: Harry <[email protected]>

Author: 4 people

MonoGame.Framework/Platform/Graphics/Texture2D.OpenGL.cs

@@ -243,13 +243,15 @@ private void PlatformGetData<T>(int level, int arraySlice, Rectangle rect, T[] d
                 GL.DeleteFramebuffers(1, ref framebufferId);
 #else
                 var tSizeInByte = ReflectionHelpers.SizeOf<T>.Get();
+                var pixelToT = Format.GetSize() / tSizeInByte;
+
                 GL.BindTexture(TextureTarget.Texture2D, this.glTexture);
                 GL.PixelStore(PixelStoreParameter.PackAlignment, Math.Min(tSizeInByte, 8));
 
+
                 if (glFormat == GLPixelFormat.CompressedTextureFormats)
                 {
                     // Note: for compressed format Format.GetSize() returns the size of a 4x4 block
-                    var pixelToT = Format.GetSize() / tSizeInByte;
                     var tFullWidth = Math.Max(this.width >> level, 1) / 4 * pixelToT;
                     var temp = new T[Math.Max(this.height >> level, 1) / 4 * tFullWidth];
                     GL.GetCompressedTexImage(TextureTarget.Texture2D, level, temp);
@@ -264,6 +266,12 @@ private void PlatformGetData<T>(int level, int arraySlice, Rectangle rect, T[] d
                         Array.Copy(temp, tempStart, data, dataStart, tRectWidth);
                     }
                 }
+                //If we want the full texture rect == (0,0,width,height), we dont have to allocate a temp buffer to copy from.
+                else if (rect.X == 0 && rect.Y == 0 && this.width == rect.Width && this.height == rect.Height && pixelToT == 1)
+                {
+                    GL.GetTexImage(TextureTarget.Texture2D, level, glFormat, glType, data);
+                    GraphicsExtensions.CheckGLError();
+                }
                 else
                 {
                     // we need to convert from our format size to the size of T here
@@ -272,7 +280,6 @@ private void PlatformGetData<T>(int level, int arraySlice, Rectangle rect, T[] d
                     GL.GetTexImage(TextureTarget.Texture2D, level, glFormat, glType, temp);
                     GraphicsExtensions.CheckGLError();
 
-                    var pixelToT = Format.GetSize() / tSizeInByte;
                     var rowCount = rect.Height;
                     var tRectWidth = rect.Width * pixelToT;
                     for (var r = 0; r < rowCount; r++)