The rounded corner calculation only took the actual texture dimensions
into account, effectively turning all pixels outside completely
transparent. Furthermore, `texelFetch()` in the default window shader
does not work with any `GL_TEXTURE_WRAP_x` parameter.
As a result, a to-be-tiled root pixmap wasn't properly tiled and only
visible in the original top-left corner.
- Changed the default window shader to use `textureSize()` to scale the
texture coordinates into the proper range for use with `texture2D()`.
- Added a new uniform to the default post-processing shader containing
the effective texture size for rounded corners calculation instead of
relying on `textureSize()`.
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