llvmpipe: increase PIPE_CAP_MAX_TEXTURE_BUFFER_SIZE value

Closes: #4237 (closed)

Increase GL_MAX_TEXTURE_BUFFER_SIZE from 65536 to 134217728 because:

• 65536 is a very small number when working with large dataset and this can block some software from rendering the data even when the hardware supports way more
• 134217728 is ~2Go of data and is a fairly common supported size (see https://opengl.gpuinfo.org/displaycapability.php?name=GL_MAX_TEXTURE_BUFFER_SIZE). Radeonsi is for example also supporting GL_MAX_TEXTURE_BUFFER_SIZE=134217728