virgl: Report CONSTANT_BUFFER_SIZE according to GL_MAX_UNIFORM_BLOCK_SIZE
GL_MAX_FRAGMENT_UNIFORM_COMPONENTS may not report a size that is useful to calculate the supported UBO size. Use the value GL_MAX_UNIFORM_BLOCK_SIZE instead when the host supports this.
Related: virgl/virglrenderer#286 (closed) Host side fix: virgl/virglrenderer!922 (merged)