When rendering to an offscreen texture for `Bitmapdata.draw`,
we first render to a temporary frame buffer, and then copy the contents
of the frame buffer back to the target texture. However, this results
in blend modes being incorrectly applied - for example, rendering with
BlendMode.SUBTRACT will subtract against the framebuffer (which starts
with each pixel as 0x00000000), instead of the previous BitmapData
contents.
To fix this, we now use our texture target as the frame buffer
when performing `render_offscreen`. This ensure that we blend
over existing pixels (taking into account the `blendMode` provided
in the `BitmapData.draw` call).
When multisampling is enabled, we use a copy pipeline to copy
the existing contents of our texture to a fresh multisampled frame
buffer (the non-multisampled texture target becomes our resolve buffer).
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